At last year's Elle Magazine Style awards, Joseph Fiennes was voted Britain's most stylish man. He laughs when I bring this up. "Actually, I'm a bit of a square," he says. This is not false modesty. He is a bit of a square. The look he's wearing today - Timberland boots, green fleece, faded 501s with ripped knee - is more 80s than new millennium man.

But Fiennes would probably take this as a compliment. So charming, so self-deprecating is he and so desperate not to appear 'wanky' (his word), you wonder if he's got bodies buried in his basement. With those huge, soulful eyes, impeccable manners and air of inner strength - this man was born to be a romantic lead.

Which is why, after Shakespeare in Love, it was so important for him to go in the opposite direction - to play a sleaze, a psycho. Someone like his character in the upcoming anarchic comedy Rancid Aluminium, based on James Hawes' cult novel.

Fiennes describes the movie as "Le Carre meets Carry On." His character, Deeny, is an Irish accountant of dubious morals. The role will have his Shakespeare in Love fans weeping into their popcorn. "Yes, I'm sure some people will be a bit disappointed," he smirks, "but then I don't want to start playing parts for other people."

So was it the sleaziness which attracted him to Deeny? "I like to leap from one extreme to another," he says thoughtfully. "So I did Shakespeare [In Love] that summer, then A Real Classy Affair at the Royal Court and then went straight in to doing Deeny, who was a fun character, a bit off the wall. It was nice to play something because it was modern, a supporting role, and he wasn't the love interest."

Is he uncomfortable with the romantic hero lark, then? "No, I'm not," he says quickly (his agent no doubt breathing a sigh of relief). "I just think it's important to mix it up a bit." Fiennes does concede, however, that with two consecutive high-profile period roles, in Elizabeth and Shakespeare in Love, he was in danger of throttling his career with a pair of tights in much the same way that Helena Bonham-Carter nearly choked the life out of hers with a corset. "It's odd," he says, "because I'd been slogging away for nearly a decade previously in the theatre. So in my mind, I didn't think of myself at all as being in a period drama rut, but when you look at the exposure film gets, it's colossal. The majority of people know you from one big success. So I'm sure that to a lot of people, I'm perceived as 'wearing tights'".

Fiennes might not have won an Oscar nomination for his role in Shakespeare ("I got nominated for a Bafta, and Screen Actors Guild, whereas the year before I was a nobody. So for me that leap was blessing enough," he says). But ever since, he's been offered all sorts of parts, including, bizarrely, "a film about a big monster in a cave in the Amazon."

Here are some facts about Joseph Fiennes. He's 29. His mother was a writer/painter, his father a photographer. His nomadic upbringing in London, the West Country and Ireland resulted in his attending 14 schools. He lives in Notting Hill and supports Chelsea FC. He's one of six siblings and has a twin brother, Jake, who works as a gamekeeper in Norfolk (the others are all involved in the arts.) He recently travelled around India on his own, and longs to go back, and he'd love to be involved in a re-telling of Herman Hesse's Siddharta ("It's such a beautiful story".)

Currently on his stereo you'll find Travis and Finley Quaye.

Apart from titbits like these and intense discussion of the acting craft, the enigmatic Mr Fiennes gives away little. And that's how he likes it. Being the focus of press attention is a burden he accepts gracefully but hasn't got accustomed to. "It wasn't something I bargained for as a jobbing actor," he says. He has a reputation for hating interviews, and, though always charming, makes you aware of where the boundaries lie. "I'm aware I'm a commodity. But I have no interest in discussing my private life, inner thoughts, other celebrities," he told one interviewer. Scrupulously honest? Or needlessly guarded? Your call. Take for example the following exchange, all delivered with a charming smile:

Heat: I don't want to go into any depth with this, but what is your current dating situation?

JF:[laughs] You don't need to go into depth about that.

Heat: Are you single?

JF: I'm currently very happy.

Heat: Are you still with the same person?

JF: I'm currently very happy.

His reticence may have had something to do with the fact that at the time of the interview, Fiennes had just ended a relationship with This Year's Love actress Catherine McCormack and begun one with Natalie Imbruglia. Or be due to seeing the tabloid battering older brother Ralph took over his relationship with Alex Kingston and Francesca Annis. Or the result of being stung himself: "It's never been that personal, but I have been followed a couple of times," he says. Then the floodgates open. "I was on holiday," he sneers, "and these two ugly MUPPETS were by the swimming pool, and then they sat behind me at dinner. In retrospect, I remember thinking: 'How fucking low can you get - sitting behind someone at dinner, and listening in?' And they took photographs and all that. It makes the whole press relationship more strained. There are wonderful journalists out there who you should trust. But you don't because of muppets like them." (Fiennes is clearly riled by this relationship talk. In an interview with The Daily Telegraph soon after ours, at their mention of a rumoured relationship with Gwyneth Paltrow, he responds: "You're beginning to sound like Heat now.")

Fiennes will have to get used to this loss of privacy, given the fact that he's tipped as Britain's biggest male star of the future. And surely it's part of the Faustian pact you make when you go into showbiz? "True," he nods. "You want a film to be successful, and you want to be appreciated by an audience. But the magnitude of it with Shakespeare was new to me and I found it extraordinary and terrifying. It's peculiar for the focus to be on you rather than the work. It makes me laugh. I find it sad that people should find any part of my life interesting. I'm really very boring." Self-deprecating and enigmatic to the last: Joseph Fiennes, international man of mystery.