The strapping, gung-ho fearlessness of Tilda Swinton is unchanged, despite recent wooing by the Hollywood mainstream. Her fondness for the abject is undiminished, unlike many top-notch actresses who, when faced with an underwear scene, demand a bottom double, a personal trainer, or some fine silk in which to perform.

Swinton relishes her position as the queen of grim and, this month, strips down to her raw essence on film opposite George Clooney in Michael Clayton. She was ever thus: I first saw the red-haired giantess in underwear in her last theatre performance, 19 years ago. She was playing to a half-shocked, half-spellbound audience at London’s Royal Court in Manfred Karge’s Man to Man – a one-woman cross-dressing play, throughout which she wore a dirty grey builder’s vest with sweat-yellowed armpits.

Vanity is alien to her. Her beauty is there to be used and abused. In the decaying industrial landscape of Young Adam, a fly was encouraged to land on her nipple during filming, making her bony, ravenous character all the more corpse-like. And once, she laid herself bare to constant scrutiny, sleeping fully clothed for a week in a glass case in the Serpentine Gallery, surrounded by gawpers.

Naturally, she was attracted to the miserable, lonely underwear scene in Michael Clayton, in which she plays an American executive gripped by vice-like ambition and desperation who is tempted to murder to save her career. Alone in her hotel room, Swinton’s character, Karen Crowder, sits before the dressing-table mirror rehearsing a corporate speech. She’s in her bra, and a middle-aged droop of flesh sags beneath the strap on her back.

“That image struck me very early on when I was reading the script,” Swinton recalls fondly. “It was one of the things that made me want to do the film – it made her seem vulnerable. She needed to have a certain kind of body, which I built with the help of rather a lot of pie.”

What Swinton recognises in these roles is that the body matters so much more to the identity of a woman than it does to a suit-camouflaged man. Towards the film’s dénouement, Crowder gets ready, laying out her sharp jacket and reinforced tights, “preparing like a Samurai warrior, her hair frozen by the hairdresser, ready for battle,” says Swinton. “I kept thinking of Condoleeza Rice…” She grins wickedly. One scene has the executive running in the gym – “but with a pot belly, which shows she’s driven, yet the control’s not working. I find that kind of detail enticing.” It’s a part lots of actresses would pay not to play.

Swinton’s method borders on madness: when she played the White Witch in Disney’s The Chronicles of Narnia blockbuster, her corset was so tight she had to be propped upright on a stand between takes. “I am a soldier,” she says, and she does come from a long line of upper-class military men. “I live a soldier’s life when I’m working. That’s how it feels to me – except I’ve got a slightly greater chance of survival.”

Her colleagues more often describe her as a trooper, someone who brings a film crew and cast together, rather than storming off to a star trailer. She is also willing to put her name to small, independent projects, often co-producing them for artistic rather than monetary rewards. Her eight years of work with the radical director Derek Jarman made her as fascinated with film-making as acting. She first appeared on screen amid the decadence of Jarman’s Caravaggio, and went on to shoot Edward II and Wittgenstein.

“Derek made film-makers of all of us. From the sound guy to the music man to the technical team, we were all responsible for the film. It was a collective – an undemarcated environment that was very pre-industrial. That was my education. There was never really a space I wanted to take up in the industrial cinema in England.”

Swinton sometimes speaks like the Communist cardholder she was when the party still existed, but, with a Coen brothers’ film coming up, she finds the mainstream embracing her. She says Michael Clayton – a legal eco-thriller – is part of a new “Hollywood Lite” genre. The film was made by the company that produced Syriana. “It’s a new way of making political Hollywood films that are not underfunded and on the skids. Once Clooney was on board, that wasn’t really a problem, of course.”

Swinton is not keen on live performance, and had an unhappy season after university at the Royal Shakespeare Company. “A play would have to be that good again to tempt me back to the stage,” she says, referring to Man to Man. She shrugs. “I don’t love the theatre. I’m just not one of them.” Her great love is film, and if she likes a director, she will play whatever part is required, just to be involved. “But when you are in every frame of the film, it’s different. It’s what I do best – the filigree, the detail. You can do a few scenes with a broader brush, but I like to be the sensibility of the film, to get inside the head of the person in the frame – that’s the alchemy I really enjoy.” Perhaps the finest example of this is Orlando, in which she switched gender exquisitely, over 400 years.

At 47, she appears less androgynous and boyish than she was then, as we sit talking in the café of the tartan-themed Balmoral Hotel in Edinburgh during the Film Festival. She is here to make a speech tomorrow as festival patron – a sign, if ever there was one, that, at some time in the past few years, Swinton has gone seamlessly from avant garde to grande dame.

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