The spectacle of Lindsay Duncan being not nice – selfish or truculent or evil to her core – will never get old. She just looks so angelic, and takes such a palpable delight in playing a character who isn’t, it’s like watching someone break out of prison.

I speak to her by Zoom. She is at home in London. She has a new ring light, which makes her look extremely radiant (I believe that’s the point), and she is really pleased with it, having just seen herself in the forthcoming comedy drama Truelove: “Let’s be frank,” she says. “It’s such a shock when I don’t look as I do in my bathroom mirror, which is perfectly acceptable.” I say, because of course I do, that she looks great in Truelove, just maybe a little grumpy, and she laughs that off. “I can’t be accused of being vain, in the respect of work. But this ring light, I think it might be a good thing.”

Truelove starts at a funeral, with a gang of friends in their 70s, who’ve known each other since they were teenagers, agreeing that however they go, they don’t want to go like Dennis, who’s just died some unspecified bad death. Tempting, then, to think you’re settling in to a show about death and (formally or informally) assisted dying, but it’s not that at all. Duncan, the lead character, is tricksy as anything: she has itchy feet in her marriage, she’s defiantly non-maternal towards her adult daughter, she’s 72 years old (73 in real life) and still smokes, for Pete’s sake.

It’s essentially a portrait of an unsatisfied woman. “She wants more,” Duncan says, with verve. “Looking at this long marriage, which isn’t about dismissing your partner, because they want to go on a cruise and you don’t. There’s deep, deep affection there. But it’s not enough.” We chuckle a bit at how taboo this is: it’s hardly a dramatic trope, the older woman who just wants one more throw of the dice. “As we live longer, as we’ve had careers, we might well get to our 70s and think, ‘Shit, I didn’t have a very interesting career, I didn’t have the greatest relationship. I don’t want to walk into the sunset without more.’ It’s really important that we’re not polishing the halo while seething inside.”

She drew Phil, this prickly creature, purely from her imagination – “I can’t speak to retirement because I’m still working,” – before adding, wryly, “Well, of course I can speak to a long marriage.” (She’s been married to Hilton McRae, also an actor, since 1985.)

If anything, the more challenging element of the drama is that Duncan isn’t a very enthusiastic mother or grandmother, and emphatically doesn’t want to downsize to a bungalow for the sake of the weans. “For people who have privilege and enough money, having children has become about identity. I’m not dismissing it – having a child if you can is one of the biggest things, it certainly is for me.” She and McRae have a son, Cal, who’s in his early 30s. “But something has shifted in your idea of yourself as a parent. Maybe the fact that society barely exists. It’s been fractured. Where are the neighbours? Where is everybody? Where are the things we used to take for granted about growing up in society?”

It’s so plain to see why she was such a favourite actor of Alan Bleasdale, of Stephen Poliakoff: she dives so instinctively between the personal and the political, the micro and the macro, the feeling and the thought. “The best writers,” she says, “create a fascinating Trojan horse, and out comes the politics. Bleasdale had an absolute passionate commitment to the politics, and mad humour; he was saying proper tough stuff, but entertaining you hugely.” His series GBH was screened in 1991, and quite a lot had happened to Duncan by then.

She grew up in Leeds, then Birmingham, with two Scottish working-class parents; her father was killed in a car accident when she was 15, and her mother, “widowed, had no money. That was not a problem. I had fantastic support from my mother in whatever I wanted to do, and I had a grant.” It took her two years to be accepted to London’s Central School of Speech and Drama – “Eventually one of them said yes. I’m not bitter, I don’t blame them” – and then rep theatre, which she describes warmly. “It wasn’t about you as an individual, it was about doing a play in a company. It might have been hierarchical, but for the most part, it was mutually supportive, and you were taking baby steps into the world.”

Top Girls at London’s Royal Court theatre was the breakthrough, in 1982: not only for Duncan, but for the playwright Caryl Churchill, and for raging, studs-first second-wave feminism. “I just couldn’t believe it,” Duncan says now, “that something so all-round ambitious could arrive on a page, and then on a stage. She took a swipe at all that shit” – the patriarchy – “and went, ‘We cannot give up humanity, as women, we’ve suffered enough. We’ve got to wake up and make this happen.’”

Duncan’s other signature role, in 1985, was Madame de Merteuil in Les Liaisons Dangereuses, which would of course make Glenn Close famous on screen a couple of years later, but was first a huge deal on stage, going from Stratford to London to Broadway.

If all that – the creativity, the comradeship, the radicalism – sounds a little idyllic, Duncan’s career took off during peak chauvinism, and she remembers doing the audition and, later, press, for Further Up Pompeii, the 1975 TV movie spin-off of the Frankie Howerd sitcom. “It was my first television. I was asked to show my legs at the audition. I was wearing a full-length dress as costume, so why should I need to show my legs? I was never a young, glamorous person at all. I honestly, honestly wasn’t. Then, later, I had a photocall for that job, where I was asked to bend forward.”

She does a Zoom impression for me, of being asked to flash her cleavage, and she’s laughing, while also recalling: “It was really painful actually, looking back. I wasn’t confident, I wanted to please people. I didn’t even know quite what was happening to me. I got back to the flat I was living in, with two guys, and just burst into tears. I carried this mix of shame and distress that I could hardly articulate. Can you imagine anybody asking me that now?” At this, she dissolves into laughter: “Well, obviously not now.” Her self-deprecation – this is a genuine new one on me, someone claiming that she was never young – should be irksome but there’s a lot of ethereal mischief in it, and I can’t help laughing.

She was never self-assured, she says, and barely knew what she was doing, least of all on TV. “I was lucky enough to have reasonably good taste in material, and I worked with some fantastic writers, and those relationships made my career what it is. But it’s not been about going upwards, it’s about the nature of it, which I would define for myself as working with writers.”

She loves working on short films. “It isn’t a career move or a money move or anything, but these people are sending their work out at the very beginning of life, it’s going to be their calling card. It’s a delicious thing.” Most of all, she loves working.

Truthfully, Duncan’s performance is my favourite thing about Truelove and recalls so many of her roles, the way she imbues them with radical restlessness. “People in their 70s,” she says, “some of that reality is about death and illness, but a lot of it is, ‘What do we do now?’” The funny thing is, she nearly didn’t take the role. “I felt about doing the job the way I’d feel about a cruise – sorry to people who go on cruises. I hadn’t read the script. I just went, ‘Four months? With a lot of old people?’ I can honestly say it’s been a real highlight in every way. But you know: a lot of old people.”

• Truelove is on Channel 4 at 9pm on 3 January

• This article was amended on 3 January 2024 to correct the year in which Lindsay Duncan played the role of Madame de Merteuil in Les Liaisons Dangereuses.