It has never occurred to me to think of the word "cut" as onomatopoeic before, but when Tilda Swinton says it, what springs to mind is a glinting knife slicing through steel. Swinton says briskly of Tony Gilroy, the writer/director of her new film, "He's no slouch", and quite clearly, neither is she: her patrician, military background is inescapably obvious as soon as she enters the room, her bearing commanding; and then there's the mesmeric, pale gaze, the famous alabaster skin, the slick of red hair, the sapling-like figure. She really is quite something.
Tony "no slouch" Gilroy is best known as the screenwriter of the three Bourne films; but next week will see the release of his directorial debut, Michael Clayton, which stars Swinton alongside George Clooney, Tom Wilkinson and Sydney Pollack. A smart, noirish thriller, as involved in its plotting as the Bourne films, but constructed with a more obvious formal precision (and without the car chases), Michael Clayton is a story about corruption. Clooney plays the "fixer" in a big New York law firm called upon to unravel the messy situation that prevails when a colleague, who is acting for agrichemicals company U/North against a group of farmers claiming they have been progressively poisoned by the company's products, has a kind of psychic crisis, and decides he is working for the wrong side. A great deal is at stake, especially for the U/North in-house attorney, Karen, played by Swinton.
Without appearing to do very much at all, Swinton offers a gripping vision of a woman whose sweaty anxiety and Electra-complex-freighted relationship with her boss are her essential components. She is a figure whose language consists entirely of corporate-speak, who projects no personality except for that communicated by her Condoleezza-inspired fashion sense - and yet whose vulnerability and fear are visible from the first moment we see her. In her first major scene, she is preparing to be filmed for a corporate video, selecting her outfit, rehearsing her lines - "and rehearsing her life, really", says Swinton. "Tony and I talked about this a lot, that she was playing this part of Karen and we have no idea who she really is. There's no way of telling through this set of signals that she is determined to project." This moment is balanced by a later scene in which Karen prepares for another big moment: "She's obviously been to the hairdresser and taken them a photograph of Condoleezza Rice and said, 'I want that,' and they've taken out this huge can of hairspray and made her this helmet. And then she's gone upstairs and laid her clothes out, like a samurai." The jacket she puts on is puffy, miles too big for her, like a ludicrous coat of armour, but it doesn't protect her - this is her "moment of collapse". Karen's story is, to Swinton, a "tragedy, in the Greek sense".
Swinton's quixotic career in the movies - from Derek Jarman collaborator in the 1980s, to the White Witch in The Lion, the Witch and the Wardrobe, via Sally Potter's genderbending Orlando and a number of roles working with American indie directors such as Spike Jonze and Jim Jarmusch - isn't really a career at all, so much as an active intelligence ranging widely over the world of cinema. Her choices are made not because she wants to work up to winning an Oscar, nor because she wants to play particular kinds of character, but because she wants to satisfy her curiosity about film-makers she admires - a film enthusiast, if you like, who just happens to be a great actor.
It is as a self-confessed film geek that she talks about The Man from London, the latest film by Hungarian director Bela Tarr, which premiered at Cannes this year. "I hope this demonstrates once and for all that there is no point in people asking me why I choose characters, because I palpably don't. I choose film-makers and not parts," she says crisply. Indeed, she's in The Man from London - an adaptation of a Georges Simenon novel - for about five minutes, mostly yelling her head off, and dubbed into Hungarian. She is not the only actor to be dubbed: Miroslav Krobot, who plays the protagonist, Maloin, says his lines in his native Czech.
"I call it medieval cinema - that's the best way to describe it," she says of collaborating with Tarr, whom she describes as having a "shamanic, mesmeric power". She says: "One of the things I really value about him is his way of working, which used to be more common. For instance the dubbing issue - Fellini dubbed all his films. I remember as a child watching La Dolce Vita, which has always been one of my favourite films, and not understanding what was this incredible frisson of weirdness between Marcello Mastroianni and his girlfriend. I couldn't work it out, and it was years later I read her lips and saw that she was speaking English: it creates a very interesting dissonance. And I now know, having worked in this way, with someone yelling at you in Czech while someone else is talking to you in Hungarian and you are shouting in English. You can't understand what he's saying, and they don't understand what you're saying and you're having a row that is founded on the fact that you don't understand each other. It makes for something very particular in the air."
There's also Tarr's particular way of shooting - each scene in one massive shot, requiring endless rehearsal and a balletic physical precision on the part of the cast and crew. "It's no picnic," she says. Each scene is actually shot to the score, which perhaps accounts for the pulsating, rhythmic nature of the film. "I remember the first time I saw the film - and I saw it very much as a Bela Tarr fan," she says. "There's a moment where Maloin walks along the wharf and the sea slaps up against the jetty, perfectly timed, and I remember thinking for a moment, 'How did Bela do that?' Then I had to say to myself, 'WAKE UP.' He didn't get the tides working for him. But everything counts in those scenes, every bird, every breath of wind, every wave lap, and that's so pure. It's a cinematic experience that I really value, as a geek."
Swinton's life in acting began on the stage, but it is film she is really gripped by, and she hasn't performed live since her famous sojourn, asleep, in a glass case in the Serpentine gallery, London, in 1995: a piece she called The Maybe. "That piece was a personal thing, a hybrid gesture, a cross between what I love most about live performance, which is everybody being in the same room together, the kinetic energy you get from that, and what I love about cinema, which is the possibility of seeing someone in an unwatched state. So I developed this gesture of the sleeping figure."
She says the stage was a red herring; had Cambridge been running a film studies course while she was an undergraduate (reading social and political science), she would have found herself gravitating towards it. But it wasn't, and nor did she have much exposure to film as a child (the home of Major-General Sir John Swinton, a lord lieutenant of Berwickshire, not being a natural home of the avant garde, one suspects) though she recalls that during the endless, lonely summer holidays she would go for long rides every day - and in her head, the cameras were rolling: "My horse used to cross over the cobbles from the courtyard into the drive, and at that moment I used to say, 'Action' and then I would be in a film."
What interests Swinton about performance, in life as in art, is the gap between what is projected and what is true. She recalls a train trip to school, when she was about 10: "I was pretty miserable, but I wasn't showing it. And I suddenly became aware that you can feel one thing and show another, you can fool people, you can never know what someone's thinking. What I realised then - this is pretty awful, but it's true, and it's good to get this digested early - is that you can't take sincerity for granted.
"There's this myth in drama, which I think comes from theatre, and I think it comes from writers, who are too articulate for their own good, that everyone is sincere and able to communicate their ideas exactly at the moment that they are having their ideas. And it's not the way it is. And you can explore that in cinema. I tend to make friends with film-makers who are interested in inarticulacy."
Jarman, who died in 1994, was a way into that, since he was a visual artist rather than a literary film-maker. She misses him, and says there has been a decade of "cultural amnesia" about his kind of art. She's working with Isaac Julien on a new film about Jarman, to be released next year. What she misses, she has written, is "the whiff of the school play" about his work. "It tickles me still and I miss it terribly. The antidote it offers to the mirror ball of the marketable ... is meat and drink to so many of us looking for that dodgy wig, that moment of awkward zing, that loose corner where we might prise up the carpet and uncover the rich slates of something we might recognise as spirit."
· Michael Clayton is out Fri 28. The Man from London is released next April
