Tilda Swinton: 'I was expected to marry a duke!'

It's freezing when I meet Tilda Swinton, in a hotel overlooking London's Leicester Square. The country is still in the grip of the grimmest winter in 30 years, not that it has dented her mood. "We were snowed in for seven days!" she says, with glee. "In the north of Scotland, it's been extraordinary. It's been like Siberia. Just to show you..."

She leaps up, pointing to her hip to demonstrate the depth of snowfall: "It was up to there..." It conjures an enduring image: Swinton, surveying her kingdom, like a Highlands version of the White Witch she played in The Chronicles of Narnia films. I question why she didn't head for warmer climes. "Why would I when there's hip-high snow in the north of Scotland?"

That comment rather encapsulates Swinton: she may have an Oscar and may count Leonardo DiCaprio, George Clooney and Brad Pitt among her co-stars, but she's extraordinarily down-to-earth. When it comes to Hollywood, she's like the party's eccentric guest. "I'm like an escapologist," she says. "I can go in for a bit, and then leave." Quite apart from helping maintain her sanity, this policy ensures she hasn't built up an entourage. She doesn't, for example, have a publicist – usually an essential A-list accoutrement. "Couldn't afford one and what would I do with one?" she says. "I don't actually know what they do. I think they're supposed to keep you in the news and I'm really not interested in the news any more than necessary. The only reason I have any interest in papers is to talk about films."

This she does very well. You don't interview Swinton so much as engage with her – particularly when it comes to cinema. As passionate about Avatar as she is about the work of Robert Bresson, Swinton's here today to talk up I Am Love, a project she's been developing for 11 years with its director, Italian-born Luca Guadagnino. Gloriously operatic in its intensity, it's an intelligent, searing study of infidelity, betrayal and love in the closed circles of upper-crust Milanese society. She plays Emma, a Russian émigré married into a wealthy Italian clan who begins a life-shattering affair with a friend of one of her grown-up sons.

Swinton performs in Italian, filtered through a credible Russian accent, and her character is, according to the actress, a "cipher". "She's someone who has lived undercover for 25 years, not even with her own name, with nothing from her own life. So she brings nothing. She's completely empty and she's completely disguised – and to be disguised that long is an interesting psychopathy, I think. I went out of my way not to make any analysis of what her previous life was. She makes the decision not to bring any of it with her. Her decision is to start a new life completely – and we meet her at a moment when that life has reached a turning point. The life of the mother, of growing children, is kind of at an end now."

She calls the film her "meat and potatoes"; it's the kind of fare she likes to get her teeth into – more so, presumably, than her forays into the Narnia-like fantasy that is Hollywood. "What I'm doing with Luca is much more my home territory – trying to get absurdly ambitious independent European films off the ground, over many decades." You only have to look back on Swinton's career, and the film-makers she's remained loyal to, to see this is not unusual. Most will point to the late Derek Jarman, the avant-garde director with whom she made seven films in eight years. But there are others: Lynn Hershman, for example, with whom she's made three uncompromising works, and Sally Potter, who cast Swinton in arguably her most iconic role – as the omnisexual Orlando in the 1992 Virginia Woolf adaptation of the same name. "People ask me to be mascots in their films," she says. "It feels like that."

In some ways as ageless as Orlando, Swinton has defied the rules of an industry geared towards youth. After spending one season at the Royal Shakespeare Company – she left, comparing it to working for ICI – she didn't get her first film role until she was in her mid-twenties, when Jarman cast her in Caravaggio. "I kept my head down when I was Keira Knightley's age," she says. "I kept under the radar, because I was so longing to be 40. I knew it. I thought, 'If I can just keep my head down, just keep playing with my friends until I'm 40, then I will have the nerve to come out.'" It's no bad thing, she estimates. "People aren't sick of me quite yet. Maybe that's a clue. Maybe it was a self-fulfilling prophecy. I was never any dolly-bird, was I?"

While that's true, Swinton is the sort of woman you can't take your eyes off. Impossibly tall and thin, her ghostly-pale androgynous face is illuminated by emerald eyes and capped by a brushed-back crop of hair (blonde, at the moment, rather than her more famous bright red). Today, she's dressed in what might be considered conservative apparel for her – black trousers and a charcoal pullover, worn over a light grey polo neck. Fashion is just one of the many things Swinton does differently. Take a look at the summer 2009 edition of AnOther magazine, in which she models a series of stunning outfits by innovative young designers, or the 2008 Oscars, the year she won Best Supporting Actress for her guilt-stricken lawyer in Michael Clayton. A red-carpet risk-taker, she simply oozed confidence in a washed silk-satin black Lanvin gown.

Remarkably, Swinton turns 50 this year (not that you'd know, her skin freakishly free of wrinkles). Perversely – though typically for her – she has no interest in such a milestone. "Is 50 more important? I think 49 is really important. Maybe that's because I'm surrounded by too many hippies." Drifting off into a digression about the spiritual significance of 49, she then tells me she's never liked playing the "numbers game". "The rules are you have to have found what you want to do with your life by the time you're 19. Wrong! Or that you have to be in a significant relationship in your twenties. Wrong! It's all just nonsense! In my life, I have to say, if I were to play that game, it's just a beeper constantly. It's like the vulture gnawing at you."

Swinton wouldn't know how to be conventional if she tried. It's almost too obvious to say her childhood moulded her this way, but it's unavoidable. Her father, Major-General Sir John Swinton, is the former head of The Queen's Household Division and Lord-Lieutenant of Berwickshire and Swinton, if she so wished, could trace her paternal ancestry back 35 generations, to the ninth century. Along with her three brothers, she travelled extensively when she was young – her father's position leading to numerous postings around the globe – yet she also attended the élite boarding school West Heath, where she was a contemporary of Princess Diana. A life of leisure, living in the upper echelons of society, awaited.

Yet if her path was set for her, she quietly rebelled. After taking two years off, working as a volunteer in a South African township and then in Kenya, she went to Cambridge to study political sciences and English literature. Was she expected to go into academia? "No, I think I was expected to marry a duke! And when it became clear I wasn't going to marry a duke, I think all bets were off, basically." So how did her family feel about her acting? "You'd have to ask them," she retorts. "I think they're incredibly fond and bemused and totally 'Wassever, no big deal!' about the whole thing. Bit late now. I don't know what they'd prefer me to do. They're wise enough people to want their children to be happy."

Still, it would've been interesting to eavesdrop on the dinner-table conversation when Swinton first announced she'd joined the Communist Party (before later switching her allegiances to the Scottish Socialist Party). "It must be tricky when a child goes into a completely different world," she muses, suddenly thinking of her own brood – 12-year-old twins Xavier and Honor – who came courtesy of her 15-year relationship with Scottish playwright John Byrne. "I ask myself what it must be like sometimes. I think if my children – my insanely pagan hippie children – eventually turn round and say that they're going to be fascist accountants or join the BNP... well it's a challenge, I think. But you know what? You just go on loving."

As far as Swinton's concerned, she's not even really an actress. I'd read a comment that she was planning to give it up for poetry, but when I mention this, she corrects me. "Other way round. I've never taken up acting and I was a poet. I went to university as a poet. I'm a complete fraud. I was accepted as a poet. And I got there and I stopped writing. I started performing very half-heartedly." So she's a poet at heart? She lets out a sigh. "My daughter [Honor] was asked when she was six what she was going to do when she grows up, which is the most awful question to ask anybody of any age. And she said, 'I am a poet.' Which she is. And I was so envious that she has this confidence to say this."

If she had a choice, Swinton would call herself "a scientist" – and she certainly experiments like few others. In 1995, the year after Jarman died of an Aids-related illness, she climbed inside a glass case at the Serpentine Gallery and dozed for eight hours a day for Cornelia Parker's work The Maybe. Almost a decade later, she took to the Parisian catwalks for designers Viktor & Rolf, strutting down the runway to the sound of her own voice, advising attendees to "Follow your own path". More recently, she contributed speaking parts to three songs on Patrick Wolf's album The Bachelor, as "the voice of hope". For the record, while she plays a bit of piano, she's not a "practitioner" of music.

Swinton also set up her own boutique film festival – renting the Ballerina Ballroom in Nairn, the Scottish seaside town close to Inverness where Chaplin used to holiday and which she calls home. The festival lasted eight and a half days (in tribute to the Fellini film) and audiences brought homemade cakes to munch on while watching everything from Polanski to Czech surrealism. Last year, she took her quest to bring arthouse cinema to the masses one step further, as she and a group of volunteers pulled a mobile cinema screen, mounted on a 33-tonne truck, for an hour a day through the more remote areas of Scotland. "We only plan to do things that nobody would think we'd do," she grins.

While swinton may live in the Highlands, she doesn't exist in a bubble. In 2008, the year she was sweeping up awards for Michael Clayton, her private life had a good going over by the tabloids. She concedes it's a consequence of an ever-growing Hollywood profile that's seen her work with such maverick talents as David Fincher (The Curious Case of Benjamin Button), the Coen Brothers (Burn After Reading) and Spike Jonze (Adaptation). "It's to do with walking into a different market square," she explains, in her curious way. "I was in the alternative hippie end of the art freaks around the corner, underneath the bus station, and I walked around the corner and went into the slightly glam market where people were selling off-cuts from Monsoon. There's a different kind of scrutiny there."

As Swinton puts it, winning a "national award" like a Bafta, which she did for Michael Clayton, meant she was suddenly fair game for such attention. "It's like having your passport checked." The problem was, those checks intensified, as her living arrangements with Byrne were put under the microscope. With the couple said to be sharing their house with Sandro Kopp, a German-born artist 21 years Swinton's junior, rumours of a ménage à trois began to circulate. As you might expect, Swinton gave as good as she got. Asked, in one particularly spiky article in the Daily Mail, if she shared her affections with both men, she answered, "That is absolutely none of your business," with the requisite confidence needed to deflect such a personal invasion.

Today, she dismisses the whole affair. "There are some people who got a bit over-excited and went up all sorts of gum trees shall we say – insanely hallucinogenic gum trees – and had to be educated. We live an extremely orthodox life. I don't any more live in a couple with the father of my children. How many countless people out there can say the same thing? We are incredibly close friends. We bring up our children together. We no longer live under the same roof. We live over the road from each other. I have been in a relationship for the past five years and continue to be in a committed relationship with someone else. The father of my children is [also] in a relationship with someone else."

Still, she does take issue with one element of the Mail's story, which quoted her as saying, "I am a soldier". She first of all concedes that being a soldier's daughter is good training for being a film-maker. "The whole thing of working from campaign to campaign, the whole strategic thing, the whole thing about working in a group, comradeship, is very good training. I do talk with my soldier brother about our respective lives, and we do find parallels." She takes a deep breath and continues. "Having said that, I never said that I'm a soldier. I would never say that. I wouldn't be that disrespectful to what that means. I'm the last person who'd say that. If I were in Afghanistan and read someone saying that at a film festival, I might be a little peeved."

As to the future, she is next set to work with Scottish film-maker Lynne Ramsay. They plan to adapt Lionel Shriver's "extraordinary" Orange Prize-winning novel, We Need To Talk About Kevin, about a mother dealing with the horror of her child being the perpetrator of a Columbine-style school massacre. She and Guadagnino have also formed their own production company, The Love Factory, named after the 1999 short they made just as they were first formulating I Am Love. There's talk of them remaking Morton DaCosta's 1958 film Auntie Mame, with Swinton taking on the Rosalind Russell role.

The truth is, Swinton's just reaching the peak of her powers right now. "I'm really grateful for this – and it's not true for lots of people – but for me, in every way, life gets better," she says. "In every way."

'I Am Love' opens nationwide this Friday