Emma Corrin asks if I’m familiar with the concept of the Saturn return, the theory that in our late 20s we experience a period of transition, of personal growth, a shift that marks the giant step into adulthood. I’ve heard of it, I say. “It’s a realigning of, like, what you want to do and who you are,” the actor, who is 28, explains. Anyway: “I think I’m in the middle of it. I’m not a massive astrology person, but I feel like I am going mad this year. Like, I’m starting to doubt everything – what I’m doing with my work, who I am as a person. Am I a good enough friend? Am I good enough partner? Am I a good enough actor? Am I doing the right thing? And suddenly all these questions have descended, and the ground that I’m standing on is so unstable and it feels like such an existential crisis.”

Corrin is sitting across from me in a pink-tiled cafe in Margate, Kent. Unless you count the occasional shiver from not wearing a jacket (Margate not as warm as you might expect of a seaside town in June) and the speed at which they speak – about 1.2 x the average – Corrin displays no outward expression of a person on the verge of crisis, existential or otherwise. What I see is gamine beauty, somehow enhanced by the spot sticker under the fringe; someone who talks about insecurity, but locks eye contact to make a fierce point. Someone who loves fashion (and is the face of Miu Miu), but is also more than happy to wear the same thing every day, like this soft-cotton red shirt and these old jeans. Someone who, when we meet a second time in London, looks immaculate but gives me a glimpse of the charming chaos of their handbag – tobacco pouch, more spot stickers, parking ticket, all spilling forth.

In Margate, Corrin (who uses they/them pronouns, but doesn’t mind if people slip) is keen for me to understand quite what a thing the Saturn return is, quite how accurately it describes what is going on in their life right now. “I downloaded this app called The Pattern, which is like an astrology app. It’s really spot on. It said: ‘This is a huge year. You’ll be questioning everything, there’ll be huge change.’ It literally said word for word what I was feeling. Which is mad.” Corrin frowns at me. “Did you have that? I feel like everyone must have it.”

I say I think so. What I don’t say is: might this palpable trepidation, this feeling that life is going to change almost unrecognisably, be linked to Corrin’s career? In the five years since they played – with eerie accuracy – Diana, Princess of Wales in Netflix’s mega-series The Crown, Corrin’s career has been on a rapid ascent. To date, credits include a sumptuous Lady Chatterley, a sophisticated portrayal of a jilted lover in My Policeman opposite Harry Styles, and a pink-haired amateur sleuth in A Murder at the End of the World. But later this month the new Marvel, Deadpool & Wolverine, is out. Corrin stars alongside Hugh Jackman and Ryan Reynolds. It’s fair to say that they will be catapulted to the sort of megastardom that is Hollywood blockbusters worth $250m. “Huge change” pretty much sums it up. Are they prepared for that level of fame? “It is very much in the abstract,” Corrin says quickly. “I’m probably living in denial.”

Obviously, we can’t talk much about the film itself – another thing about these blockbusters is that they are a blizzard of NDAs – but, briefly, for those unfamiliar with the franchise, Deadpool is a satirical superhero, who breaks the fourth wall and sends up the whole premise of the comic book genre while also conforming to it.

Corrin’s initial thrill at being offered the part of bald supervillain Cassandra Nova was dampened on meeting the director Shawn Levy. Levy’s first words were: “OK, bear with me, but we want you not to play her as a villain.” “I was, like, what? Are you serious? After all that?” But what Levy saw in Corrin was the ability to play something subtler than an off-the-peg, “pick her out of a lineup” very bad person. What he wanted was unpredictability. He wanted sunny, sunny, sunny, then – he’d snap his fingers – clouds coming in. He shorthanded the switch to: “Change the weather!” Corrin says their references for the part included Gene Wilder’s Willy Wonka and Christoph Waltz’s portrayal of a Nazi officer in Inglourious Basterds, because: “He doesn’t need to pretend to be evil. He just is.” So, the Nazi can sit at a table drinking milk and being friendly, and it’s even more chilling.

Corrin began to shave their scalp so that the bald cap would fit snuggly, which “wasn’t as bad as I thought it would be”. In fact, “I found it liberating, I got really into doing it whenever I could – when I was on holiday, even when I was on a boat.” (Once in a hotel in Italy, they blew a fuse while using clippers, causing a power outage. “Half my head was shaved and half wasn’t.”) Today, Corrin’s hair is cropped and a little bed-flattened (we have met for breakfast). There is a loose, coltishness to the way they arrange themselves on the chair, and many times in the course of our conversation Corrin uses a range of facial expressions – thoughtful pout, querying frown, intense gaze, uncertain bunching of one side of the mouth – to convey emotion that might otherwise be uncomfortable.

Corrin is in Margate with “friends” – and prefers not to discuss their partner, the actor Rami Malek. Margate appeals, not least because no one recognises them – “Or they do and they’re just not bothered, which is even better.” I ask if they have ever met the artist Tracey Emin, Margate’s homegrown queen, and Corrin’s eyes light up, because not only have they met Emin, Emin has become a great friend. “She came round once, and she was like … ” Corrin mimes Emin arms folded, looking around their house in London, eyebrows raised, head nodding, “ … because I’ve got a lot of her prints everywhere. I just love what she does. She was that voice [of a generation] and she kept fighting.”

Ordinarily, Corrin is based in Hampstead in west London, flat-sharing with three friends, including a political journalist (“a great way to stay informed”), and their cockapoo, Spencer. There seems to be a quiet effort to emphasise the unstarry nature of their existence – most of their friends are not actors, which “helps” – “I am really grateful for that.” They chat about “muddy” music festivals, about their efforts to crack the Rubik’s Cube and how Corrin was nearly there and put it down for just a moment when someone else – God damn them – picked it up and scrambled it again.

For a while, the actor was a cold-water dipper at Hampstead ponds, but, “I’d be lying if I said I’d kept it up. I was going through a lot at one point, like two years ago, and I needed the shock of it. Now just the idea of being cold … I’m too tired.” They still use the Parliament Hill lido for a hot shower because their boiler keeps breaking. “I feel an absolute fraud. All these women coming in red-faced from the cold water and being like, ‘Oh, fantastic!’, and I’d have just crawled out of bed pretending that I’ve done the plunge.”

Corrin was an “unknown” in acting terms, not that long out of Cambridge University, when called in for a “chemistry reading” during the search for someone to play Camilla Parker Bowles in The Crown. Their agent stressed this was not an audition, but they were asked to read for the part of Diana, too. Eight months later, the call came to audition for that role in front of Peter Morgan, the show’s creator and writer. Corrin perfected Diana’s slow, lilting voice by rehearsing with their mother, Juliette, a speech therapist.

They have previously described the attention the role bought as a “poisoned chalice”, “grim”, “inescapable” and “overwhelming”. They said: “I remember there was this one scene we were filming outside [Diana’s] flat when she’s leaving for the last time, saying goodbye to her flatmates. We had loads of supporting actors being the press, and then beyond the cameras are film cameras as well – actual paparazzi. And it was such a weird double world. I was like: no acting required.”

Since then, Corrin’s young adulthood has been spent relaying from set to set. While negotiating all the attention, they were working out aspects of their gender identity, coming out as non-binary in 2021 and changing their pronouns first to she/them and then to they/them. They told the New York Times: “My identity and being non-binary is an embrace of many different parts of myself, the masculine and the feminine and everything in between.”

We find ourselves back on the subject of change, the chrysalis period, when Corrin tries to persuade me that they have “second puberty” acne, which may be a real thing, but pretty hard to discern in their case. Their skin is like glass. The actor carries their own skincare to shoots (Augustinus Bader, since you ask) and has an “amazing” dermatologist who told them: “People – I think women specifically – go through a second puberty in their late 20s, which no one talks about. Hormonally and physically, it’s exactly the same as the one you go through when a teenager.”

Hormones bring us to the subject of children, which is “probably” related to “this biological shift”, but not something Corrin has thought about “hugely” (except they would like to write a children’s book, “probably not under my name”). And then the subject of children brings us to the state of the world – who would want to bring up a child when the world is burning, literally and metaphorically? – and we dwell here, Corrin asking me if I think that all generations have thought the world was about to end. “This is when I’m like: 100 years ago, did people say the same thing? When we were on the cusp of the first and second world wars? Were they saying the same thing? None of us would be here if it was as bad as they thought, right? That’s the thing: does everyone always think there’s going to be catastrophe? I don’t know.”

Corrin has always been a worrier. As a child, their mother would wake up to find a fort of their stuff under the living-room table in preparation for being washed away in a flood. But they feel now, more than ever, the threat to civilisation. With so many issues being urgent and catastrophic, “it’s completely impossible to know what to fight for, where to place your energy and your care”.

Social media “especially feels like a swamp”. They worry how they might protect any children they may have from this potential for massive harm. “I hear parents on Radio 4 talking about how there is no filter on what children are watching. The government is doing nothing about it. The way that algorithms work means they are now feeding kids stuff they shouldn’t be seeing. That would terrify me [as a parent].”

Corrin’s own relationship with social media is “very conflicted”. They would like to step away from Instagram, where they have 838k followers. “I want to pretend that I could just leave and it would be fine. But I know that for a lot of people, particularly younger than me, it is their salvation. It’s a real way of connecting with people. Especially with some of the topics people follow me for, specifically around gender.”

Corrin recounts how young people have approached to thank them, or tell them they have inspired them, or that things Corrin has said have helped them come out to their parents. “Parents of non-binary or trans children, too,” they add, recalling a festival where they were approached by a parent inspired by their portrayal of Virginia Woolf’s gender-fluid Orlando at the Garrick theatre in 2022. “That’s when I find it hard, because I don’t want to ever remove myself from a space that might be helping people, even in a small way.”

They squeeze their fists and press them to their face. “But, at the same time, social media makes me very anxious. And there’s this other whole question: I want to be authentic on it, but the platform itself is inauthentic. So, I have a very conflicted relationship to it. And I don’t know the answer.” The anxiety runs further. Corrin wants to be a “good person”; a good role model. But fears being straitjacketed by one thing, one “self”, on the platform, and therefore constrained; unable to struggle through changes humans naturally experience under such intense scrutiny.

“When I was younger, I always thought that when you reach 25, you’re done. You’re an adult. You are who you are and you are completed. When you’re a kid, all adults seem pretty homogenous, right? But now I think you’re never finished. And that excites me. That’s what I find interesting about writing and storytelling: that people are constantly changing and it never stops. You are always questioning everything, changing, growing, insecure, falling in love, out of love, like it’s endless. And that’s beautiful.”

A seagull pads into the cafe, yellow-beaked and looking round with curiosity. Corrin greets it with childlike delight. They have always been “obsessed” with animals, having grown up in Kent (“not countryside-countryside, but there was green space”) with two brothers, Richard and Jonty born three and six years later. The family shuttled back and forth from South Africa, where their mother is from, and where their father, Chris, worked as a businessman.

Corrin describes their mother as a “child whisperer”. “She’s got this endless imagination and makes up characters and people and stories and worlds. I grew up among those stories.” They were convinced that fairies existed. “My mum being this amazing world-spinner was, like, ‘Leave a note for them in a milk bottle. See if anyone responds.’ And they did. I was obsessed! I started writing every day. One morning, there was a letter: ‘Dear Emma, we’ve had to move. We’re not going to be here any more. Love, the Fairies.’ And I was devastated. My mum was like, ‘I can’t keep this up. It’s too much pressure.’”

While still super-close to their brothers, four years ago Corrin had a revelation about their relationship to their parents. “The scales fell away and I saw my parents as people with faults and limitations and insecurities for the first time. You feel like you move to this position of being their equal, and then you move sort of beyond that. You realise that sometimes you actually become their parent as well. There must be a kind of deep-down break inside when you see your parent at their lowest. You realise that you need to be there for them in the way that they were for you.” Corrin is making alternate rotating gestures with their hands. “It switches the dynamic. But it also means you become their friend in a way that you haven’t before. And there’s, like, an openness and a respect for them that hasn’t been there before.”

They describe how, on those hours on layovers in airports, their mother encouraged them to create narratives for the people they saw hurrying past to another life. Corrin would write those “airport stories” in exercise books. “I remember one about a rugby team and one being about an old man. What I’ve learned, maybe, is that it’s not so much about eavesdropping or being interested in their actual lives, it’s about what they could be going through. The unsaid.”

Corrin says they always loved acting, and remembers in primary school being complimented by a parent for their role as Toad in The Wind in the Willows and told: “You could be an actor.”

“It was kind of a perfect complement of imagination and feeling like I was doing something for people, entertaining people, which I enjoyed. And absorbing and escapist.” They add, laughing: “I had a brief period where I really wanted to be a marine biologist, but I’m terrified of the sea. So that was very fleeting. Such random fears!” Isn’t the sea a famous sexual metaphor? “Is it? Oh God, what does that mean? I’m scared of sex? Hahaha. I think it’s fear of the unknown. I’m fine with shallow water, but it’s like deep water. Where anything could be swimming around, sneaking by. I can’t … sharks. No.”

I ask if Corrin was a happy child. “I found ways to be happy. But I think those ways were very much not in the real world. It wasn’t automatic.” Their therapist once told them: “‘You know, it’s no surprise you do what you do.’ I was like: ‘Excuse me?’ She said: ‘Well, you know, you don’t have to be yourself. You don’t have to … You can happily be anyone but yourself.’” They laugh, saying it was savage, even for a therapist.

So, does Corrin think their therapist is saying that they are uncomfortable with being themselves? “No. I think it just means that there’s some refuge in telling other people’s stories when you’re still trying to make sense of your own. I guess it is why people escape into writing or painting. It’s a way to express stuff and live in a world that’s not necessarily you. I think my therapist is probably spot on.”

Then Corrin wants to make a serious actorly point about something the director Michael Grandage told them on the first day of shooting My Policeman. “We’d done a couple of scenes, and he came and found me in my green room and said: ‘Well, I just want to say, I’ve never been more surprised.’ He said: ‘I was worried, Emma, I won’t lie, that your brain seemed like everywhere except where we were: in rehearsal. I’ve never seen anyone drop in like that when the camera went on. You were present!’ He said he’d experienced it once before with an actor in theatre. That actor said, ‘I find everything else hard, but I find this the easiest thing in the world.’ And I think that’s similar [to me], maybe. I struggle, maybe, with life generally.”

What causes those dark times? There’s a tilt of the head. “Probably insecurity. Expectation. Fame – fame is such a weird thing. Also: loneliness, loss. I think as you get older there’s a fear of losing people. The reality of losing people. Losing yourself.” They make an expression like a question mark. One of the big decisions Corrin has recently made is choosing their roles more carefully. Until this moment they have been grateful for the sheer volume of offers, by the number of scripts flapped in their face. “You feel like you should say yes to all of it. And I think it’s taken me a minute to realise that actually you don’t have to. And that it’s better to be more selective and to do those things justice. I’ve always been quite a work, work, work person. I think I’ve hit a point where I actually just want to slow down a bit and really choose carefully and specifically, and do one or two great things a year, not four.”

Corrin has said before that the film industry can be quite aggressive in terms of pushing actors to work extraordinary hours, but today says: “Maybe demanding is a better word than aggressive.” The issue is that “unless you draw really clear boundaries, [the industry] will take advantage of you. I think it’s probably more women and non-binary people.” A pause. “To be honest, I think it’s probably just me. I know a lot of people who are really good at drawing boundaries and being assertive, I’m just not one of them. I struggle with it. Because I’m always anxious about letting people down.”

I look down at my questions and see that I was going to ask how they would describe where they are in their life right now, but we appear to have covered that. Corrin laughs. “Yeah. Like in a massive period of change and growth, which is exciting, more than anything. I think it’s exciting to question things and to be uncertain, because you know that you’re going to come out of it with some new semblance of who you are and what you want to do.” Their laugh morphs into a grimace and a look of dread. Well, they have a bit of time left. According to The Pattern app they don’t come out of the Saturn return until spring next year.

• Deadpool & Wolverine is out in cinemas on 25 July in the UK.