It's not the sort of admission one could have expected to come from Stevenson a few years ago. Described variously in the past as "staunchly feminist", "intense", "feisty", "independent" and any number of other words implying plain difficult, taking a role on account of a flouncy wardrobe would have been a horrific concept. But while Stevenson's description of the aristocrat Amanda in Private Lives could easily have stood for herself once - "she's anarchic, makes up her own rules and is deliciously arrogant," she says - Stevenson has changed. Defiantly unmarried at 42, she lives with anthropologist Hugh Brody (expert on the Inuit, part-time filmmaker and her partner of five years) and their four-year-old daughter Rosalind. But the drive with which she approached acting and politics has mellowed as family life has taken priority. "Part of me thinks it's extraordinary to be part of 'normality'," she says, looking, ironically, every inch the middle-class mum in acres of beige. "There's not a morning when I don't take my child to school and think, 'I've become somebody who gets stuck in traffic jams at 8.35 every morning, drops their kids off, chats with other mums and walks off down the pavement'. Motherhood is like sex and death: you can't imagine it until you do it. It's a completely overwhelming, all-else obliterating passion for a little blob! It's like being in love all the time. You'll do absolutely anything."
Though obviously in raptures with what she constantly refers to, rather grandly, as "the family", one senses that, had there been no Hugh and Rosalind, Stevenson would have embraced singledom just as fully. It's a self-sufficiency borne, perhaps, of being sent to Berkshire's Hurst Lodge boarding school, aged nine (her brigadier father was posted abroad and didn't want her education disrupted) which, in adulthood, has matured into a willingness to flout convention. It's made her, in recent years, something of a role model for ambitious thirtysomethings now also grappling with difficult choices. Does she think of herself as inspirational? "I'd like to think people can relate to me," she says evenly. "I never set out to do things this way, but I think if you know what you want to do and you find success, kids and family are bound to come later. I don't know anybody who's had kids aged 21! God, I was far too busy getting involved in stuff, travelling around the world, experiencing life."
There's something of the zealot in Stevenson. The actress Fiona Shaw saw it, observing "Juliet lives her life as though she has just won it on the Pools". What seems to define Stevenson is a need to do. When acting was her life, she researched her parts to extremes, once trekking around the Sierra Nevada to understand her role in Lorca's Spanish folk tragedy Yerma better. And having gone to the Medical Foundation for the Victims of Torture for her role as Paulina in an acclaimed 1991 production of Death and the Maiden, she remained committed to the cause - last year she picketed with Chilean mothers in Leicester Square for Pinochet's extradition. Most recently she fronted a TV Kosovo appeal with Jill Dando.
Now her cause celebre is her family. So, she happily explains, part of the incentive for doing rep at the National Theatre is that "if you do a West End show, it's a killer for your home life. You never get a weekend away, you're never there to put your kids to bed." She's turned down two Broadway plays in the last year because they would have played havoc with Rosalind's schooling; decreed July and August no-go areas for work, because she, Hugh, Rosalind, and Hugh's two teenage sons from his previous marriage spend every summer in Vancouver. She also claims to be delighted that a Hollywood movie recently fell through, as she didn't want to wrench the family away from its routine.
Her familial tenderness is very endearing, but it's not a side of her we're used to. Even Philip Franks, now directing her in Private Lives, talks tactfully of her "insane perfectionism" and claims that friends who work together do so "at their peril". Yet it's equally apparent that there is a sizeable gap between Stevenson's aloof, luvvie public image and the more amenable reality.
"I don't know why it started," says Stevenson. "I'm not a bitch!" Joking, she narrows her eyes and shoots a withering look. Then pauses. Sighs. "Well, I say that. But in my twenties I did mouth off about things. But a journalist paints a colour on you, and that colour gets stronger and stronger so you become either very, very blue or very, very orange. You know, Anthony [Minghella] wrote Truly Madly Deeply for me, because he said, 'I'm sick of hearing them talk about you in that incredibly mean way? I know you're a nitwit and I'm going to write you as a nitwit'. And that was a lot of the reason for writing it. To give me an opportunity to be a different kind of media personality." It seems to have been on Philip Franks' mind too. "One of the reasons I wanted Juliet in Private Lives is that she's always seen as so solemn but she has a wonderful sense of humour," he says.
Media images come and go, but the actress seems to have let down her hackles. Whereas she once claimed to adore the anonymity of shopping unharrassed at Sainsbury's, she is now drawn to TV roles because they touch so many people. Following the success of Cider with Rosie at Christmas, she has signed up to star in four two-hour dramas for ITV, based on the bestselling thrillers by Frances Fyfield. "I love the thought of doing something popular," says Stevenson, excitedly. "I love stuff on TV when it means you get into a taxi or a bus or a swimming pool and someone comes up and says, 'I really liked you in this' and you have a conversation about it. Otherwise, you make a piece of work and you've no idea what anyone thinks of it."
And what should people now make of Stevenson? No longer "ferocious", can we expect her to be serene and happy from now on? "Don't know about the serene bit," she laughs, as she gets yanked back into rehearsals. "But happy? Yes."
'Private Lives' is playing at the National Theatre from 13 May.