"The nature of film is incredibly vampiric," says Katrin Cartlidge. "After making a film, you feel drained of blood. But theatre is sustaining - the people in the audience give me something back, and at the end I feel stronger. Film is just a celluloid wall - you give and you give, but by the time people react to it, it's too late, you've needed a blood transfusion. I like to bring myself to the vampire film, I have this masochistic streak. But I also think it's important as an actor to recharge. That's when I come back to theatre."
This week finds Cartlidge making one of those periodic returns to the stage, to star in Boy Gets Girl, Rebecca Gilman's third play for the Royal Court. But it's for her cinema work that Cartlidge is best known. She was the poster girl for Mike Leigh's Naked, and lent a sardonic edge to his Career Girls. She brought a self-possessed spirit to Milcho Manchevski's Oscar- nominated Before the Rain and Lars von Trier's Breaking the Waves. Most recently, we've seen her in Lodge Kerrigan's Claire Dolan, playing a prostitute who is struggling to sustain a relationship and forge a new life. It's not surprising that she finds making films a draining experience: she has a knack for rooting out tough roles - characters who are desperately insecure, or trapped by circumstance in unforgiving societies.
Cartlidge hasn't worked in the theatre since February 1999, when she played Simon McBurney's lover in Mnemonic, Complicite's investigation into memory and identity. Despite the enthusiasm, she can't help betraying nerves. "I don't think I'm a born actor," she admits, "and I find theatre acting immensely difficult - I'm very frightened about it. The sort of things that I'm capable of as an actor seem to be more valid in film."
Her part in Boy Gets Girl is certainly demanding. Gilman's previous plays have unflinchingly depicted a teenager coerced into murder and a woman confessing to racism. Here, Gilman tackles the experience of being stalked: Cartlidge plays Teresa, a journalist in her late 30s, who goes on a blind date but decides against starting a relationship. The man none the less bombards her with flowers and, when his advances are rejected, turns violent, causing her life, sense of identity and self-confidence to unravel.
Cartlidge has herself been stalked - "although it never got aggressive" - by men who pretended to be in films so that they could meet her. She is fascinated by the idea that society, particularly the celebrity culture of cinema, cultivates stalkers. "Everybody has an element of stalking in them," she says. "Every time you idolise someone you've never met, you're getting some way into the mind of a stalker."
You can tell by the way Cartlidge sinks her teeth into the issues peripheral to her characters that she takes her choice of roles seriously. The similarities between her characters are intriguing. She gravitates towards flinty women, coolly strong on the outside but bruised and vulnerable inside. "I'm not as tough as the characters I play," she says. In fact, everything about her is softer: she is chattier and quicker to laugh, her north-London accent is less sharp, even her face is gentler and less angular than it appears on screen.
Her approach to work is intuitive, she says, and what attracts her most is intelligence. "I love to play characters who are intelligent, whether that's a streetwise quality or someone who's well-educated. Intelligent women are complicated." Cartlidge herself struggled when she was at school, thanks to undiagnosed dyslexia. As a teenager, acting was a means of expressing herself when words failed her.
She picks her projects with equal care: "I try to work with film-makers who I feel will produce something original, revealing and provoking. If something provokes a reaction, it's well worth doing," she says. In fact, "provocative" has been the key word of her career, ever since she caused uproar at London's Riverside Studios two decades ago, playing a woman forced to do a striptease. "I remember that moment in slow motion - taking my top off and seeing dust rising from the audience's heads. I didn't know how to cope with it at that age, so I carried on limply. But I developed a real understanding - that the better I did the striptease, and the more confident I was as an actress, the less that reaction happened."
Her confidence was further bolst-ered by stints working as a life model at the Slade, while taking life-drawing classes so that she could appreciate the nature both of looking and being looked at. From that controversial striptease, through the garish nymph Sophie in Naked, to the prostitute Claire Dolan, much of her career has built on her unselfconscious attitude to nakedness, drawing her to explore women's relationship with sex.
"I think it's important for people to be capable of looking at the human body without feeling that it's shameful or disgusting or exploitative. I've never taken my clothes off, either in a play or a film, where I felt that the context in which it was placed was in any way titillating. But I'm not insulted by the sexual act on screen, ever. I'm insulted by the violent act. I'm interested in the animal side of human nature: when I look at what's happening in the world, I don't think we've evolved that far from the animal kingdom. It's important to go back to the drawing board in how we describe ourselves."
Cartlidge turned 40 this May, and you have to wonder whether roles that allow her to explore characters through their sexuality will keep coming. One thing that attracted her to Boy Gets Girl was Teresa's age: "It's wonderful to get a play where a woman of over 26 and under 60 is the central role." She feels that Gilman's play is part of a wider movement, a subtle change in writers and directors' preoccu pations. "Many independent film-makers and people in the theatre are beginning to see that the age range between, say, 35 and 45, up to 50 and beyond, is actually the most rich and interesting aspect of a woman's life."
In a sense, Cartlidge's career so far has allowed her to live a few years behind herself - she has consistently played women some years younger than her own age. Time is inevitably catching up with her: she would like to have children, for instance, "but I know I've left it quite late. If it happens, fine, but if not, I'll get on with my life." Her attitude to work is just as serene: "Hopefully, when I'm 45, I'll get to play women of 40! I won't be afraid of my body changing, and I hope there are artists around who are interested in the nature of getting older. If there aren't, I'll have to think again about how to express myself."
For the moment, however, she is keeping busy. US audiences can currently see her in the hit horror From Hell, which finds Johnny Depp investigating the Jack the Ripper murders, and next year she will work with Lars von Trier again, on Dogville, which will star Nicole Kidman.
These are both unusual projects for Cartlidge, who tends to avoid making films alongside major Hollywood actors. "The majority of people I've worked with have been auteurs. I find it wonderful when people write and direct their own material - there's an intensity and clarity that I love. When stars are involved, it can tend to become star-oriented, and lose its integrity. But Lars von Trier is involved in a certain amount of subversion of those values, and I think this is going to be very exciting."
Before that, we'll see her in a BBC film, Surrealissimo, about eccentric Spanish artist Salvador Dali. She plays Dali's wife Gala, and positively fizzes when talking about the role: "I had a ball; I just loved it. It was all my childhood wanting-to-be-an-actress fantasies: I got to be glamorous and bitchy, funny and vulnerable." The last time she remembers being "glamorous and bitchy", she was playing an ugly sister in a modernised Cinderella for Channel 4. Things are clearly looking up.
· Boy Gets Girl is at the Royal Court, London SW1, from tomorrow. Box office: 020-7565 5000.