Special delivery

Olivia Williams did her bit for Cool Britannia by beating thousands of applicants to play Kevin Costner's love interest in The Postman. The film fizzled but it still served as career Viagra for Williams: she's since been Bruce Willis's wife (The Sixth Sense), Bill Murray's girlfriend (Rushmore) and Antonio Banderas's forbidden fruit (The Body, a film so unremittingly bad that Williams is astonished I've seen it).

Hollywood glory notwithstanding, she's still an unpretentious London girl with an enduring love for traditional theatre. Shakespeare and Chekhov are where she's at, so what's she doing in John Osborne's The Hotel In Amsterdam at the Donmar? British theatre's original Angry Young Man may have joined the canon long ago, but he's a long way from the Bard.

'Well, exactly,' says Williams. 'I was in Love's Labour's Lost in spring, and I'm a variety addict. You couldn't get much more of a crazy spectrum than this - plus I've been laced into corsets three times now and I didn't want to stay there.'

She is both intrigued and frightened by the sparseness of the play, which explores the dynamics between three couples on a drunken weekend escape: 'It's just four things to sit on, six glasses and a script.'

And, she point outs, being Osborne, it's a damned good script. 'What possesses me is the dynamic of six people in a room. Who irritates or fancies who, the sidelong glances and the politics.'

She's equally in love with theatre and film; it's like comparing strawberries with typewriters, she exclaims when asked which she prefers. But the script is definitely the thing. Sometimes, that bias fails her, resulting in dog-like thriller The Body, but she's philosophical. 'I got to spend four months in Israel during the last intifada, and my research included helping an archaeologist exhume a 2,200-year-old body.

It was an extraordinary experience - who cares if the film was bad?'

Williams's irreverence is a long way from contempt. She loves her profession and she's winning the battle with the demon luvviedom. She has an English degree from Cambridge, an unashamed preference for traditional literature and a formidable CV, but listening to her salty, honest chat, it's the dangers, not the glories, of acting that come across.

She tells stories against herself: going up for the Asda advert and being told the way she tapped her back pocket was too sexy; determining to give up acting by 30 and being saved, literally, by the bell when Costner called; mucking up a run-through for the other five cast members on her current production.

When I ask for advice for a friend in a similar situation to hers - bright, pretty, dedicated, will give up acting at 30 - she's brutal: 'Unless Costner calls in the next few weeks, forget it. What happened to me was so bizarre, and if it hadn't I don't think I'd have worked as an actor again. It's a job designed by the devil.' Surprising words from Cinderella, but Williams is a realist.

When I ask her opinion on the Faustian pact - Hollywood glamour for artistic credibility - in which the Donmar, particularly, specialises, she's equally blunt. 'When I was in the rank and file of the RSC, of course I thought it extremely unfair that people with box office pull got the good roles. Now I think it's marvellous.

'No, seriously, it's depressing - not everyone can cross over, some of the greatest actors can't get film roles. I'm not complacent; I know I wouldn't be talking to you now if it weren't for Kevin Costner.'

Well, good for Kevin: giving us Olivia may well be his greatest service to the industry so far.