Tilda Swinton: The mother of reinvention

Tilda Swinton's latest role is that of an 'ordinary' mother who'll do anything for her children. She talks to Fiona Morrow about her exploration - on and off screen - of the chaos that blood ties can bring

Tilda Swinton is having her photograph taken. In the space of five minutes she has inveigled practically their entire life stories out of the photographer and his assistant. It's not so much nosiness as an apparently pathological inquisitiveness. No sooner has she made off with their biographies than she moves on to their equipment and the relative merits of 35mm and digital.

For her, it's the former all the way: "If you can see the picture you have taken and decide that that one will do," she comments, thoroughly engaged in the debate, "you're unlikely to push yourself and the medium just that bit further. You run the risk of never attaining your best shot."

The photographer asks her to smile and she exclaims, shocked: "Oh no, I never smile for photographs, that's not what you want." And she adopts her most intense, serious stare. But she's soon caught out when, encouraged to talk about her toddler twins, she recounts a funny moment and lets out a peal of laughter. Her strange but beautiful features are transfigured.

Snap. And Swinton is caught on an old-fashioned camera, with no hope of checking the image. "Oh dear," she smiles, adding, rather amused, "they'll never use that one, anyway."

Later, when we are settled with a pot of Earl Grey and Swinton is munching on a cheese toastie, she makes another reference to her image. She scrapes her newly shorn hair back severely from her face, exposing brittle, pointed features and a pair of protruding ears. "My alien properties weren't exactly required for this film," she smiles, very knowingly.

Swinton has, until recently, been part of the intellectual, highbrow avant-garde of British film-making, best known for her gender-bending turn in Sally Potter's Orlando and her collaborations with the late Derek Jarman. She played male on stage to great acclaim with Man To Man, and exposed her post-partum body for her part in Tim Roth's The War Zone.

If set the task to come up with her least likely role, it's doubtful if anyone would have dared to imagine her shagging Leonardo DiCaprio in Danny Boyle's disastrous The Beach. It wasn't an auspicious beginning for Swinton's break into the mainstream. Nevertheless, she dove straight back in for another go. The Deep End – David Siegel and Scott McGeehee's follow up to Suture – is Swinton's tour de force. She's playing normal, ordinary and suburban, and manages to invest her performance with enough suppressed emotion to have you reeling from the cinema, desperate for air.

A remake of Max Ophüls's The Reckless Moment, The Deep End updates the story to have respectable mother-of-three Margaret (Swinton) cover up what she assumes is her teenage son's murder of his older, gay lover. Suggest that Margaret marks a departure in Swinton's career, and she wrinkles her nose.

"It's funny," she answers in her perfectly enunciated, fully rounded tone. "I'm just so contrary, I can always see both sides of the same coin. On the one hand, yes, there are elements of departure, for example the very fact that it deals in the kind of realism that it does; the calibration of the performance – we're dealing in a kind of ordinariness that I haven't tended to explore before; and that it's a damn fine script by literate beings." Sheadds quickly: "Which is not to say that film-makers I've worked with previously have not been literate but, and they may be the first people to admit it, they were not writers and that wasn't what the projects were about."

She pauses, cocks her head as if taking part in an argument with herself, and continues: "But having said that, at the heart of it, it feels like business as usual to me – there are so many through-lines. The work with the character who is dealing with the mask, dealing with this whole question of 'How is it possible to exist in an integrated identity?' 'How is it possible to make friends with all my virtual selves?' seems to me to be my theme tune basically, from Orlando, Female Perversions, Edward II, Man To Man... Integrated schizophrenia is where I'm headed, and that's why I like to look at that idea in performance."

I tell Swinton that on first viewing I'd found the film a little cold, but then completely reversed my opinion second time around. She nods briskly: "The third time it's even better," she suggests. "I was having a conversation with myself just this morning about how easy it is for clever people to be clever, but what a particular and radical project it is for clever people to be emotional. And it seems to me that we've all become very used to clever work generally being clever only, and identifying emotional work as schlocky... In film terms particularly, people equate emotion with sentimentality."

There is, she insists, nothing sentimental about Margaret's position: "Let's talk about the mother's lot," she says, leaning forward and clasping her hands together. "The juggling of plates, the wall-to-wall, the no one's ever going to give you a medal. The never, ever a day off. The never, ever a lie in. If you have a child with eczema [as she herself has], the never, ever an unbroken night's sleep in three-and-a-half years. And on we go. And yet I don't know how it's possible to keep going. Because before you have a child you're tired all the time, and after, you think 'What on earth was I doing that made me so tired?'

"People ask how I've changed, having children. I don't think it's that you change, but that a new being comes into existence, this other self. The second you become a mother, you have to say to your former self – that individuated, occasionally solitary, rational, intellectual self: 'I'm going to have to leave you for a little bit, and I will get back to you, but now I have to become the leader' – in my case of a pack – 'I have to become an indentured slave.' And the trick is remembering to get back in touch, but so many women don't. And Margaret Hall is one of them – 17 years later, she turns round and she hasn't stopped for a minute. And that is the plain, unexotic fact."

Swinton is clearly enjoying herself, and it's not just the opportunity for grown-up conversation while the kids are out of sight. The night before we meet in Edinburgh, she arrives at the film's festival screening in a tightly tailored black suit, fishnet tights and death-defying stilettos; she looks every inch – and she's damn tall – a star.

"Oh, this is bright lights, big city for me," she laughs. After her twins were born, Swinton upped sticks and headed for the hills – the Highlands of Easter Ross where, she says, her toddlers run feral: "The Highlanders – particularly where we are, north of Inverness in the wilderness – generally know how lucky they are. And if you want to join, too, then they respect your choice. It's a relatively easy sect to join.

"It's astonishing how quickly you come off the urban drug. It's a centrifugal thing in London, it really is. For the last few years I lived there, I might as well have lived in a cave – I've seen many more people who live in London since I left it."

A far cry from Margaret's Lake Tahoe home, then. "There aren't so many people in Easter Ross in love with the bourgeois dream," Swinton smiles. "And it's that dream that's really on display in the film. Those modest, what the Americans call homes..." she breaks off with an ironic sigh. "Terribly modest and simple, made of..." She looks up and frowns, aware that there's something wrong with what she is about to say, "...clapperboard." Another frown accompanies a brisk, "Or whatever it's called."

Swinton is so extraordinarily precise in her speech, it's weird hearing her stumble. I mutter an unasked for "clapboard", and she grins: "That's spending too much time on set, you know!"

The digression doesn't affect her train of thought, however, and she quickly returns to the point: "They look modest, but I think those houses cost millions, something like $8m.

"The film is very precise about things like that." She goes into character: "Oh, you've got a lovely baby." "Oh, you think he's beautiful? You should see his photograph." Then she assumes her real voice. "It's all about taking photographs of the Thanksgiving dinner. It's all frame it, frame it, frame it. And that's what you end up getting – a framed life. It's all about a framed life."