Picture who might play the fictional alter ego of journalist and author Caitlin Moran, and a teetotal Jewish non-smoker from Los Angeles probably doesn’t spring immediately to mind. Yet Beanie Feldstein is exactly that and about to start shooting the lead in the film adaptation of Moran’s 2014 novel How to Build a Girl – the semi-autobiographical story of working-class Wolverhampton teenager Johanna Morrigan, who moves to London in the 90s to reinvent herself as a music journalist and “full-time lady sex adventurer”.

It will be 25-year-old Feldstein’s first lead role. She made her acting debut aged nine in the ABC sitcom My Wife and Kids and went to theatre camp and appeared in school productions during her teens. Her first significant film role was in the debauched college comedy Bad Neighbors 2 alongside Seth Rogen and Zac Efron and she made her Broadway debut in last year’s sellout revival of Hello, Dolly!, starring Bette Midler. But it was her performance last year in writer-director Greta Gerwig’s coming-of-age comedy-drama Lady Bird that confirmed Feldstein’s arrival. Playing Julie Steffans, the best friend of Lady Bird (Saoirse Ronan), she ran away with most of the scenes she was in. The film was nominated for five Oscars.

“As soon as we saw her in Lady Bird, we knew we’d both found Johanna and seen someone who was going to be a superstar,” says Caitlin Moran. “From the moment you see her, you root for her.

“In real life, she’s only one inch high [slight exaggeration – she’s 5ft 5in] and made of eyes. You desperately want to put her in your pocket so you can just bring her out at parties and have everyone go: ‘But this is the most luminous, talented and charming thing ever.’”

Feldstein’s real first name is Elizabeth and the nickname Beanie came from a British nanny who called her “Elizabean”. “Sometimes I introduce myself to people and their reaction reminds me that it’s insane that my name is Beanie,” she says down the phone from Los Angeles. She’s from a tight-knit, semi-showbizzy family: her father’s a music-business accountant, her mother is a costume designer and her brother is the actor Jonah Hill. Eldest brother, Jordan, a music manager, died suddenly from a heart attack six months ago, aged 40. I’m told this is the only subject that’s off limits during our interview.

If Feldstein’s grief is still raw, you wouldn’t know it because she’s effervescent company. She’s into nail art (“It’s the thing I spend the most money on”), Harry Potter (“At my core, I’m Mrs Weasley but that’s partly because I’m a huge Julie Walters fan”), Gilmore Girls and Queer Eye.

Tell us how you got the How to Build a Girl part? Were you vetted by Caitlin Moran herself?
I didn’t meet Caitlin until I’d finished the two days of auditioning. She wasn’t in the room while I auditioned but she watched them all. I remember calling home after having dinner with the whole team [Caitlin Moran, director Coky Giedroyc, producers Alison Owen and Debra Hayward] and I was floating on air. I said: “Mom, I don’t know if I’m gonna get the part, but either way, I got to meet these talented, kind, inspiring women.” So when I found out I’d got it, I was a wreck.

Were you already a fan of her work?
I wasn’t very familiar before going up for the role. I’ve since devoured every single piece of writing and every YouTube video. Once I started, I couldn’t stop.

Now you’re heading to Wolverhampton to learn the accent …
I am! I’m going for two weeks before we start shooting to just immerse myself in it. I’ve been to London twice but never the Midlands. I came with my mom on a sightseeing trip when I was younger, then again for a few days when I auditioned. I can’t wait to spend more time in the UK.

So let’s talk Lady Bird. Has it been a nine-month whirlwind of award shows and acclaim?
It’s been so special – not only because so many people have seen the film but how they hold it close. You get the sense it’s truly beloved. The movie was a turning point for me, too. I’ll remember my life as pre-Lady Bird and post-Lady Bird.

Were you a fan of Greta Gerwig’s previous work?
Massively. Frances Ha! [Gerwig’s 2012 film] was very formative for me and my friends in college, so I was excited even to be reading Greta’s script. When I got to the Julie character, I was flabbergasted by how perfectly written she was. I understood her instantly. She felt like my girl.

Julie’s become something of a cult character. What’s her appeal?
I think people relate to the fact that she’s unwavering in her love for Lady Bird – but equally unwavering in her self-worth. Julie doesn’t feel the need to change herself because Lady Bird’s pulling away from her; she stays true to who she is.

In some respects, is the film a love story between Lady Bird and Julie?
Absolutely. Greta always says that’s the big romance of the film. In most coming-of-age movies, it’s a guy arriving at the girl’s house to take her to the prom. In our film, that scene is between two girls. A teenage girl shows up for her friend, saying, “I was wrong, please have me back and let me be your date.” As the major romantic gesture of the story, that’s so meaningful.

Is it true you kept notes on your phone called “Things Greta said today”?
Well, I wanted to remember the experience as best I could – but it was an old-school notebook. We didn’t have phones on set, which was incredible – and certainly something I’ll copy if I ever direct. It forced everybody to look up, talk to each other, not get distracted. That was so vital for me and Saoirse forming our relationship because we just sat, talked, quoted Kristen Wiig sketches and goofed around between scenes.

You’ve called Laurie Metcalf, who played Lady Bird’s mother, “the most inspiring woman in existence”. Why?
I grew up only ever wanting to do theatre, that was my first love, and Laurie is an institution in the theatre world. She’s devoted to stage work but she navigates between that, film and TV. That’s a path I’d love to follow.

Metcalf is best known in the UK as Roseanne’s sister Jackie. What did you think of the sitcom’s revival being cancelled after Roseanne Barr’s racist “joke”?
That tweet was horrible. She did not deserve to be the lead of the show after that because it was the most disgusting thing.

Is it true you congratulated Greta Gerwig for her Oscar nods by buying her a bag of Cheetos and a Diet Coke?
We called it “the Greta” on set because that’s all she ever asked for. So I was trying to think what you buy for someone when they’ve been Oscar-nominated and decided to get something that nobody else would. I was lucky enough to have breakfast with her on the morning the nominations were announced. She opened the bag, started sobbing and was like [tearful voice]: “You got me the Greta!”

Lady Bird and How to Build a Girl are both female-led films – as is your next release, Olivia Wilde’s directorial debut Booksmart. Is that a conscious choice in the #MeToo era?
There’s such a push now for everyone to be represented, regardless of gender, sexuality or race. It’s immensely important to tell their stories. I’m a huge feminist, I majored in sociology at college and I care about what I put into the world. I feel very lucky in my short working life to have been part of projects I’m proud of. Greta has set the bar high and now everything has to live up to Lady Bird. That movie opened my eyes to what film can do and how powerful it can be for people.

Lady Bird is set at a Catholic high school in Sacramento. How did it compare with your own school days?
I was very much in the theatre crowd, so the fact that I was doing a high-school movie where my character was in a Sondheim musical was like living my dream. Personality-wise, I lie somewhere between Lady Bird and Julie. I have Julie’s affectionate nature and intense loyalty but Lady Bird’s fire as well.

Wasn’t your own prom date Ben Platt, star of the Pitch Perfect films and Broadway?
It was. Ben is my soulmate, the twin I don’t have. We met at a bar mitzvah, we’ve been best friends for over a decade, and even now, we still sometimes say the exact same thing at the exact same time, then just start laughing. Maybe we were separated at birth.

Jonah Hill is your older brother. Are you competitive?
The only thing we compete about is impersonating our mom. We’re always arguing over whose impression is best. Other than that, we’re so supportive of one another. There’s also a pretty big age gap, nearly 10 years between us, so we were always at different stages of life.

Might you work together one day?
I’d love to. Jonah’s becoming such an incredible writer and director, so I could be in something he created, or we could act together. We’d both be up for that if it was the right project.

Vogue called you “the next Bette Midler” after you appeared alongside her on Broadway. How was that?
An honour. Bette’s the hardest working person I’ve ever encountered. She never takes breaks, never sits down, she’s a force of nature. To be around that and soak it up was a precious gift.

Aren’t you an Abba fan?
Big Abba fan. Huuuuge. I wake up every morning to Take a Chance on Me as my iPhone alarm – it really gets you going. I’m excited about them reforming and excited for Mamma Mia! Here We Go Again. This is a very thrilling Abba year for me.

Lady Bird is available to stream now and on DVD/Blu-ray from tomorrow