Juliet Stevenson is absolutely tiny. She is a little pocket person - slim, short, petite in every way. She is not at all what I expected. I've seen her give so many towering performances that, in my mind, she was a big tall person. Yet here she is, all tiny and freezing, quietly drinking soup from a polystyrene cup.  She is, in many ways, unremarkable. I don't mean that as an insult, but if I saw her walking down the street, I might not recognise her. That seems incredible to me, how easily she would blend in with the crowd because, during her 30-odd years as an actress, she has pulled some pretty powerful performances from that tiny frame.  I first came across her when she took the lead in the 1991 anthony Minghella film Truly Madly Deeply. She played Nina, a woman grieving the sudden death of her beloved boyfriend. She spent the first third of the film crying big tears and snorting back snot flowing from her nose. It was ugly and beautiful at the same time. Then she won an Olivier for her portrayal of Paulina, a former political prisoner who has been raped, in the play Death and the Maiden. I saw her in that too and was blown away. I've also seen her in Shakespeare, Ibsen, Bend it Like Beckham and TV's a Place of Execution.  She is here, in the freezing cold rehearsal space in North London, to prepare for her next role of Dr Diana Cassell, a leading academic in earth sciences, in The Heretic, a new play by Richard Bean. "Well, she's a geophysicist who studies climate change and she won't jump on the bandwagon about the whole issue. She looks at everything scientifically and she rattles people's cages because she is at odds with the orthodoxy." Does Cassell, as played by Stevenson, not believe in climate change then?  "She believes in going her own way," Stevenson says firmly. Then, suddenly, she looks rather surprised. "It's a comedy, actually. Didn't I say that?  It's very funny. It's not banging a drum or anything."  Stevenson says that she wanted to do the play partly because she loves performing at the Royal Court in London and partly because Stevenson's character is a single mother with a teenage daughter. "I could really relate to the relationship between a mother and her teenage daughter."  Stevenson has two children: Rosalind, 17; and Gabriel, 10. She also has two stepsons - Jonah, 22, and Jomo, 26 - whom she adores. "I get on so well with them and they've been so fantastic."  She has been with her partner, the anthropologist Hugh Brody, since they met at a supper party in 1993. "It was instant. Really it was," she says. "We were at a friend's house and he was opposite me and I couldn't stop staring at him. He must've thought I was mad, but after supper we ended up talking to each other and that was it."  She says that she had had her fill of actors by then. "I had certainly dated a few and I'd had enough really."  Why does she think that was the case?  "Well, I think I was looking for someone who wasn't in the industry because for most of my twenties long-term relationships and being a mother weren't on my mind. I was so focused on my career when I was younger. It was everything to me. I'd do lots of research, going off to Russia or wherever. It just doesn't work if two people who are focused on acting get together."  Until Stevenson had children she says she was resolutely undomesticated. "There was nothing in my fridge but mouldy yoghurt. I actually went to friends' houses just to get fed." What did she live off, then?  She laughs. "apart from other people's charity?  No idea." She did, however, have some vague notion that she wanted to be a mother. "It was there, I suppose, a latent maternal feeling, but I couldn't have done it in my twenties. I really wasn't ready for it. I was enjoying my life too much."  Her daughter shows every sign that she wants to be an actress. Is she happy about this?  She thinks for a bit. "It's not up to me though, is it?" she says. "She's a very grounded person, very emotionally intelligent. I think she'll make choices that are right for her." She concedes that Rosalind finds Stevenson's life glamorous. "Well, when I go to awards ceremonies and the like. She likes seeing me work, she loves coming to the theatre, but I tell her that it's not as glamorous as it looks. It's hard work."  She describes her life as one big rush. "The children tell me we have a sitting room in our house, but I call it the run-through because I am always rushing off from one place to the next. I do the kids, I work, I organise childcare. I wash uniforms, sports kit. There never seems to be enough time."  Her husband travels a lot. "He has spent the past 12 years working with the Bushmen of the Kalahari and he also does research at a Canadian university, so he goes backwards and forwards. "When I'm working he holds the fort at home. It's a juggling act for both of us." Does it bother her that he travels so much?  She looks surprised. "No," she says. "It's what he does."  It's something she is used to because she has spent much of her life on the move. Her father was in the army and the family travelled all over the world. "as a child I never stayed anywhere for more than about two years. It didn't bother me, but now I have my own children and our two stepchildren I can't imagine how my parents did it. There's all that packing up to do. It's almost impossible to think about it." It also meant that she never put down any roots. "I didn't really have any friends because I was never anywhere long enough." She became quite a solitary child who lived in her head.  Stevenson began to act when she went to boarding school, Hurst Lodge in Berkshire. "I had a wonderful teacher who was Welsh." It was this teacher who encouraged her to draw from her own experiences. "I hadn't realised that about acting then, the depths you could go to. For example, I had never really enjoyed poetry but she got me to read auden and my life changed."  at first, Stevenson didn't enjoy school. "I was homesick at first. There were no computers in those days, no mobile phones, so contact with my parents was only through letter-writing. I found it very awful. I was 9."  It wasn't the school's fault. "Oh no, I loved the school. We had a ball, us boarders." But at the end of holidays she would dread going back. "I'd have this awful sense of loss," she says. "I have used it a lot in my acting. God knows, I have other places in which to look for loss. My brother in a car crash died just before my son was born, and he was only 48. My father has died. You can't