Kelly Reilly has had it with hoodies. “I've gone from being a normal woman who cares for children to someone who will kill any kid that comes near me!” she says. “There are no f***ing boundaries any more.” Reilly is describing, with near breathless excitement, her onscreen journey in the provocative new British horror flick, Eden Lake. Here, as a schoolteacher on a romantic mini-break at an isolated lake in the Midlands with her boyfriend (Michael Fassbender), she is tormented and ultimately tortured by a gang of feral teens. Midway through the movie, however, she boasts the above-mentioned Rambo moment, where she lathers herself in muck, rips shreds from her summer dress, arms herself with a shank, and fearlessly faces her foes. “It's a nightmare fantasy that asks, ‘What would you do in this situation?'” she explains. “What does rage do to you, and how far would it make you go?”

The movie is an unusual choice for the 31-year-old actress, a delicate olive-eyed beauty who is more famous for her theatrical roles (the youngest Olivier Award nominee - for the 2004 Donmar Warehouse production of Miss Julie) and a leftfield arthouse stalwart (see Nic Roeg's recent Puffball, or Cédric Klapisch's Russian Dolls), than she is for being a pulp movie starlet. And yet, she adds, Eden Lake is serious, too. “There's a very real truth to this movie, about the kids out there,” she says. “We have to recognise that these are our kids, and we're responsible for them!” She stops herself and, worried that she's sounding too earnest, adds, “It's also, first and foremost, a genre thriller, and it stands up with the best genre thrillers out there.”

Which might explain the movie's second act obsession, amid regular drenching, dousing and slathering, with Reilly's cleavage. “What can I say? I am a woman, I have cleavage, I was in a dress, and I was running around,” she says, “What am I supposed to do, tape them down?” No, but the dress is kind of, well, plungey. “I chose that dress to make her look as vulnerable as possible, like Alice in Wonderland,” she says, adding wryly, “But thank you, anyway, for publicly acknowledging my breasts.”

Reilly, of course, has impeccable credentials (no puns intended). She was inspired to act, as an aimless adolescent, by reading the complete works of Chekhov. The daughter of a policeman father and hospital receptionist mother, she was encouraged by the drama teachers at her local comprehensive, Tolworth Girls' School in Kingston, who took her to an inspirational West End production of Brecht's The Resistible Rise of Arturo Ui. At 16 she got an agent. Two months later she was cast opposite Helen Mirren in Prime Suspect (she played a young tearaway). Co-starring roles in TV and film followed, but it was in theatre that she came of age. She garnered awards kudos and rave reviews as Elaine opposite Kathleen Turner's Mrs Robinson in The Graduate, as Debs opposite Matthew Perry in Sexual Pervers-ity in Chicago, and as Desdemona opposite Ewan McGregor in Othello. The dramatist Terry Johnson, who directed Reilly in The Graduate, even wrote an entire play, called Piano/Forte, just for her (it was subse- quently produced, and starred Reilly, at the Royal Court).

“This recognition in theatre, the opportunities and the high profile, is a nice situation for me to be in, but it's not necessarily the truth,” she says, explaining that being perceived as a board-treading doyenne can have its limitations. “It's not as if Michael Bay is coming to me, offering me a role in Transformers 2 and I'm saying, ‘No darling, don't you understand, I'm in theatre!'” she says, before surveying a movie career that has often steered stridently away from the mainstream. “But at the same time I have high taste values, and I don't want to see myself in a piece of s***.”

This dilemma, it seems, will soon be academic, as Reilly is next starring in Triage, opposite Colin Farrell, as the wife of a photojournalist who goes missing on assignment, and after that she'll play one of Hugh Grant's two prospective lovers in the seriously commercial Working Title comedy Lost for Words. Of the latter movie she purrs demurely, “It's a very nice role, and I'm very, very, excited about doing it.”

In the meantime, Reilly has more important things on her mind, such as where to live. She is currently bouncing between London and New York, “like an international gypsy” with her fiancé, the Israeli actor Jonah Lotan. She says that for an actorly couple, they're surprisingly mellow. She adds that she's keen to put the whole ‘actor' thing in perspective anyway, and that she's mildly New Agey, likes to meditate and can think of nothing worse than being on the cover of Heat magazine. “I'm just an actor, for God's sake,” she says, trying to sum it up. “I don't want people to project stuff on to me, I don't want to be someone who thinks they have all the answers, and I don't want to sound self-righteous about my opinions.” She then adds, one last time, for effect, and for truth, “I'm just an actor.”

Eden Lake is released nationwide on Friday

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