The start ofthe affair

Do you mind?" Maggie Cheung waves an unlit cigarette through the air and wriggles out of her leather jacket. Given the all clear, she flips open her Zippo lighter, enjoying its satisfying clunk, and snuggles into the sofa - the epitome of unpretentious chic in a T-shirt and simple cotton skirt with a diamanté heart tattoo glued to her calf.

Do you mind?" Maggie Cheung waves an unlit cigarette through the air and wriggles out of her leather jacket. Given the all clear, she flips open her Zippo lighter, enjoying its satisfying clunk, and snuggles into the sofa - the epitome of unpretentious chic in a T-shirt and simple cotton skirt with a diamanté heart tattoo glued to her calf.

She is in London for a couple of days to promote her latest collaboration with director Wong Kar-Wai, the extraordinary, mellifluous melodrama In The Mood For Love. She plays Su Li-Zhen, a woman with an adulterous spouse, who gradually falls in love with her neighbour (Tony Leung), whose wife is sleeping with her husband.

"I hated the film. I hated how I looked; the way I moved." Cheung is nothing if not frank about her reaction to the film's premiÿre at this year's Cannes Film festival. "I think we were all very upset after that first screening," she recalls. "I walked out, I was really quiet - I couldn't say anything. And Wong Kar-Wai could see that I was unhappy, but there was nothing he could do."

It was hardly surprising that the finished film came as something of a shock: it had taken a year to film, and Cheung had been in the dubbing theatre looping lines just a week before the Cannes screening. "We had shot so much footage that wasn't used - they had enough material to tell the story in 15 different ways," she explains. "And almost everything in the finished film was shot in the last few weeks - it felt as though we could have made it in a month or two."

When filming began, the only thing that Cheung knew for certain was that the movie would concern an affair. "We had talked about working together a few years ago, but it didn't happen then. I was dying to work with Wong Kar-Wai again so when he called and said he was ready, I made sure I would be available," she says.

It was a far cry from their first association, when the actress did everything she could not to have to work with the director: "That was his first film - As Tears Go By - he had been a scriptwriter, and I didn't know him. I was very busy in those days, the Hong Kong film industry was at its peak and we were all shooting about five films back to back," she pauses and smiles to herself, absent-mindedly picking up a chocolate biscuit. "I really resented the fact that he insisted on using me, but his financier and my manager were friends so I had no choice - I said, 'I'll give you eight days, that's it'."

She soon realised, however, that here was a different kind of film-maker: "I had never thought of acting as a serious career," she explains. "I was just having fun and I knew that playing the stereotypical pretty girl in a film wasn't going anywhere - you know there will be a younger, prettier girl along soon - but I planned to just take the money and go."

"But Wong Kar-Wai, the way he worked with me, the way he opened things up for me - he told me that acting is not about expressions, you do it from the heart - and I took everything more seriously from that point onwards. So in many ways for me, he is my saviour."

Cheung pulls her legs up underneath her, and takes another biscuit. She is completely at ease, the mark, perhaps of a woman who has packed a lot into her 36 years. Born in Hong Kong, she came to Britain with her family when she was eight, and lived here until she was 17. On her return to Hong Kong, she came second in the 1983 Miss Hong Kong beauty pageant, taking a job as a television presenter as a result. She made her first film a year later, and now boasts a filmography of some 75 movies. She's worked with Jackie Chan ( Police Story), Stanley Kwan ( Full Moon In New York, Too Happy For Words), and played herself in Olivier Assayas' Irma Vep (later marrying the director). This is her fourth film with Wong Kar-Wai, the others being, As Tears Go By; Days Of Being Wild and Ashes Of Time.

She cites In The Mood For Love as the most demanding: "Physically I've been under more pressure before, with action scenes, or accidents, or very fast shoots, but emotionally and mentally this was a hard journey. I was away from my husband for a long time; it was very frustrating."

She had hoped to play one of Wong's messy romantics: "Usually Kar-Wai likes the scruffy side of beauty - people with mussed up hair and unwashed clothes and I was looking forward to that," she sighs, lighting another cigarette. "And I'm really against playing anyone too beautiful because I don't think people look further than the surface, they don't follow your emotions" she continues, alternating drags on her cigarette with bites of chocolate digestive.

In fact, her character is a model of repressed self-denial, who wears her exquisite cheongsams, high-heels and make-up like a shield, to protect herself from the vagaries and vulgarities of those around her. "She needs it to hide her insecurities," Cheung concurs, wrinkling her nose. "But from the moment they did my hair I was stiff, and by the time I was wearing the clothes and the make-up I didn't feel very human. And I couldn't get a handle on this woman, so uptight about everything and such a chicken."

At a second viewing in Cannes, Cheung let go of her expectations and watched the film Wong had edited rather than the movie she had played out in her imagination over the protracted shoot: "I enjoyed it that time, and all the elements began to sink in. I realised that Kar-Wai had been very clever in making the film a very simple story of two very simple people. Because you never really see what happens behind closed doors, the film carries this incredible intensity. I think Kar-Wai had been blocked throughout the shoot because he was worried there had been so many films about affairs and he wanted this one to be different. He wanted to do it in a completely new way."

She puts her feet back on the floor, straightens up, and adds, "I had been disgusted by this character, I couldn't figure out who she was, why she would walk the way she did, and William Chang Suk-Ping, the editor and set designer, said to me: 'Why can't you accept that a woman like that exists? Is it just because it's not you? Why shouldn't she walk like that?' And things began to fall into place for me: I realised that my main hang-up was my performance. I felt that I had put in a lot of effort, that I had tried, but that I wasn't good, and so I felt that I couldn't be a very good actress."

That she should feel that way is a shame, I tell her, because her performance is quite stunning yet wonderfully subtle, conjuring perfectly the pain of betrayal and the heightened sensitivity brought about by the suppression of desire. She shrugs, unconvinced: "I no longer think that I am bad in it, I just don't think that I am good."

Her next project is with Hou Hsiao-Hsien: "I've known for a year that I would be playing a DJ, so I had imagined drugs, nightscenes, smoking, being bad," she tells me, relishing the list of transgressions. "But I've just read the synopsis and she's like a health freak - a herbalist vegetarian who meditates and does yoga."

Cheung laughs, reaching instinctively for her cigarettes, just as someone arrives with an enormous cream-laden cake for the PR whose birthday it happens to be. "Mmm," she says, licking her lips. "It's like that TV advert that was on here when I was a kid." She sucks in her cheeks, puts on her best Kenneth Williams' voice, and bellows, "Naughty but nice."

'In the Mood For Love' is out on 27 Oct