JOSEPH FIENNES is not one for capitalising on success. He treats it rather like a childhood illness: mystified as to how he caught it, but convinced that when the fever is over, a period of rest and recovery in a darkened room will probably do the trick.

"When I'm pressured by people who say, 'Well, while you're hot, you ought to follow it up', my reaction is to do the opposite. I don't feel in a hurry. I don't feel any urgency. I guess I just don't have that ambition to push on for the sake of it."

Instead, he pushes off. After Shakespeare in Love, he worked for £200 a week at the Royal Court Theatre for a month. Then, when the film fever was at its height, he went on a Buddhist retreat to Canada. "It took me six hours to drive there. It was in the middle of nowhere. When I arrived at this wild place in the hills, I was met by a young Buddhist novice who recognised me from Shakespeare. Not something I gambled on. I was a bit depressed."

Losing part of your privacy, he shrugs, goes with the job. "It was naive of me to think otherwise." But he reminds himself there's no need to overreact. "If Salman Rushdie can go about without a disguise, I'm sure I can. It makes me more determined to carry on buying the paper at the local corner shop."

Fiennes is limpid to his fingertips, casual to the point of undress. The laces of his Timberlands are undone, jeans stylishly ripped across the thigh, facial hair neglected, eyelashes so long it must require real effort to blink. Is it possible he does not know how glamorous he is?

The slow, upwardly mobile right eyebrow, used to good effect when he wants to convey amused scorn, suggests otherwise. When I ask him whether it was true that he and his Shakespeare co-star, Gwyneth Paltrow, had fallen for one another off screen as well as on, the eyebrow soars in reproof. "You're beginning to sound like Heat magazine now."

Chemistry . . . he pretends not to know what this means. It can happen on screen between two actors who can't bear each other, he argues, just as love scenes between people who are madly in love can be a complete flop. "It's just part of the magic of celluloid," he says. "It helped that we [he and Paltrow] got on very, very well. I was nervous. I had just had one or two bit parts in films and several years of theatre and I was very excited to be alongside someone of such . . . notoriety in film. She was warm, generous and giving."

He is self-consciously guarded about his private life. "I might have found someone . . . but I am going to keep it ambiguous," he says. His last serious known girlfriend was the actress Catherine McCormack. He says he finds it terrifying that the average length of a marriage is now three years and expects nothing less than a lifelong partnership himself: "I think I'm a bit of a romantic."

Yet, when I met him, Fiennes had just taken six months off work, culminating in a trip to India, alone. He was planning to "disappear to the Indian Ocean" for Christmas. "I love going without telling anyone, just catching a flight and disappearing, with no plans and a bit of cash. Travelling alone is the only way to travel. You don't have to make decisions, you don't have to compromise."

In India, he avoided the Taj Mahal and the rest of the tourist beat in favour of rambling among the 10th- and 12th-century temples of Khatjurahu. "The relief work was a mixture of ancient Hindi tales about Shiva and Ganesh and other gods, but a lot were explicit Kama Sutra positions. The women all looked like Lara Croft, with perfect breasts and hips and thighs, doing all sorts of things. Very funny and incredibly beautiful."

He does go on sociable holidays, too - "if there's enough stimulus - I get restless lying on a beach". Friends say the resulting group snapshots always show Joe trying to look ordinary and melt into the background, but somehow jumping out of the picture, as if coated in ectoplasm.

The director of Shakespeare in Love, John Madden, says Fiennes doesn't have to turn on the charm and sensuality, he just has it. Flocks of besotted girls would agree, though "openness, raw anguish and vulnerability", might not be their reasons. Confronted with the alarming possibility that Shakespeare may have made him a sex symbol, he goes off at a tangent about escapist cinema in India, adding: "I am aware I'm a participant in the history of escapism."

Fiennes is 29, and although he has served a seven-year apprenticeship in the theatre, his screen fame has been achieved at hurtling speed. Cate Blanchett, the Queen to his courtier in Elizabeth, lauds him as "gifted beyond belief". Tom Stoppard says he is not your run-of-the-mill heart-throb: "He is sensitive with a very macho centre."

Fiennes says he will never be able to shake off the actor's basic insecurity because "everything is impermanent and subject to change", but you don't sense, in his case, that the threat is very real.

Shakespeare in Love received a fistful of Oscars, but there wasn't one for his Will Shakespeare. Did he mind? Of course not. "I just felt happy. It was such an amazing episode in my life. Previous to that, I was battling in the market place for a job."

Now he can afford to take six months off while job offers pile up unopened. "I finished a film in America in the summer and it's quite easy to get seduced into doing one after another. I wanted a bit of distance and to understand why I was doing it, what was my passion behind it." He spent his travels "just reading and generally getting away from the craziness of this business".

He was sent a video of Shakespeare, but he hasn't watched it. "I couldn't bear to. It feels odd to settle back and watch a film you are in. It's strange enough to hear one's voice on the radio, let alone see yourself on screen."

No account of Fiennes is complete or properly comprehensible without the starry litany of his gifted siblings. His older brother Ralph (nominated for Oscars for both Schindler's List and The English Patient) is still the better-known actor; his sister, Martha, directed Ralph in Onegin, brother Magnus is a musician, sister Sophie a producer and brother Michael also an actor. [Note: Michael Emery is an archaeologist, not an actor.] Although his twin, Jacob, is a gamekeeper, Joseph makes no distinction between his job and theirs.

"He is doing what he passionately believes in, as we all are. That is the common thread: pursuing our passions and all throwing our guts into what we believe in. We are all very close. I have grown up with them, they're like best mates."

The children had a peripatetic upbringing. Their father, Mark, was in constant pursuit of the next photographic commission and their mother, Jini, was a painter and writer. "I think it boiled down to money," says Fiennes. "There were seven children to feed and photography was not the most lucrative profession. My parents' dream was to bring their children up in a rural environment. Sometimes, they were forced back to London, or wherever the work was, so they could never quite realise this."

The seventh child, the illegitimate son of an Irish Catholic girl, was fostered. Jini saw an advertisement in a national newspaper that read: "Michael, aged 10, urgently seeks a home where he is allowed to read a book", and the earth-mother in her responded immediately.

Fiennes attended 14 different schools. As he suffers from mild dyslexia, the confusion must have been intense. "I felt we were odd because we were always moving and everyone else had a permanent base. I never knew what the syllabus was, but in other respects, it was great. One could reinvent oneself at each school as a different person. In Dorset, I adopted a Dorset accent; in Ireland, I had an Irish lilt. Socially, it makes you more sophisticated. We all have that instinct to fit in."

He claims he was "a terror and a bit of a bully" until he was about 12, but there, sadly, his waywardness ended. "I don't know what happened, but I skipped adolescence. I rather envy people who broke out when they were teenagers. It would have been something to look back on."

Far from being some bohemian idyll, his early years were "a mad, messy, smelly, noisy, chaotic adventure". His foster brother, Michael Emery, has recalled "a lot of weirdness" as well as warmth in their family life. The boy was evidently upset by Jini's unpredictability, particularly her tendency to throw things, and eventually he felt the need to cut loose from the noisy, hyper- talented Fiennes family and to make his own way as an archaeologist.

Jini died from breast cancer six years ago when she was only 55. What was she really like? Fiennes is non-committal. "Pivotal, like most mothers."

He left school at 16 to study art but soon succumbed to the theatre. After three years at the Guildhall School of Music and Drama, in London, he went straight into the camp gothic ghost story, The Woman in Black, in the West End. "Becoming an actor was like being picked up by the scruff of the neck and being plonked in the right place." But at first he worried about "moving in on" Ralph and being such a bad actor that he would embarrass his brother.

After several low-profile seasons at the Royal Shakespeare Company, he began to attract attention five or six years ago - for his Jesus in Dennis Potter's Son of Man and his Belyaev in A Month in the Country (with Helen Mirren and John Hurt). Films followed - Stealing Beauty, Martha - Meet Frank, Daniel and Laurence, then Elizabeth and Shakespeare in Love. "I just learnt the lines and hit the mark," he claims.

Next, Fiennes opens in a small-budget British screen adaptation of James Hawes's Rancid Aluminium, which he describes, in an unusually sustained burst of dialogue, as "a sort of tongue-in-cheek le Carré meets Carry On. It's a high-octane thriller about a hopeless young man who does too many drugs and has to take on the firm of his father who has just died. His sperm count is very sad, very low, he's trying to get his girlfriend pregnant, the business is going bust and he is bailed out by the Russian mafia, and this is set up by his very dodgy accountant."

The thought of so much roguery, of catching up with the self that never broke out, pleases him. He ends triumphantly: "I am the dodgy accountant." Rancid Aluminium will be released nationwide on January 21.