In the forthcoming film Shakespeare in Love, there is a scene in which an emerging young playwright pursues the object of his desires by boat down the Thames. As he steps into the boat, the boatman does a double take, hesitates, then says, "Are you an actor?" The playwright, played by an emerging young British actor, rolls his eyes skywards. "Ye-es," he snaps, simmering with and frustration.

The British actor is Joseph Fiennes, and we are meant to believe the playwright is Shakespeare (not that we do, of course, and herein lies the film's success). It is a question Fiennes the Younger will have to get used to answering. His Will Shakespeare has all of his brother Ralph's intensity, but is tempered and leavened with a kind of febrile, edgy intelligence and no small amount of comic bite. It's this impassioned energy that's been missing in Fiennes's earlier film appearances. If you'd watched him in Stealing Beauty (he plays the small but crucial role of the boy who finally relieves Liv Tyler of her virginity) [Note: that was not Joe's character] , Martha - Meet Frank, Daniel and Laurence or Elizabeth you'd be forgiven for thinking Fiennes only has three gears: lover, hangdog, and hangdog-lover. Casting him opposite the beautiful but always bland Gwyneth Paltrow, director John Madden (Mrs Brown) mercifully keeps the number of his leading man's limpid-eyed gazes down to a minimum, coaxing from him a career-altering performance.

I met him in the over-furnished lounge of a west London hotel. My first impression was that he's smaller than you might imagine - compact and lithe in the way male ballet dancers can be. The second is that he's incredibly shy and ill at ease. Dressed in a black vest-top, black combats and blue New Balance trainers, he sidled into the room sideways as if he was the waiter, or would rather be. He was also eager to please and very nice, lacking that patina of self-assurance - that conviction that I'm-the-star-of-the-production-I-like-to-call-my-life - most actors have. I positioned my tape recorder near him and asked him if it would put him off. "It will," he admitted, fidgeting with his cropped fringe. "But don't worry," he added hastily. "It'll be fine."

Seeing Fiennes in the flesh precisely one hour after watching him on screen, running about Elizabethan London in a state of high passion, was like seeing a blackboard wiped clean. He folded himself into the corner of a sofa, crossing and recrossing his limbs around him as if I might, at any moment, leap over the table and attack him. Although, at 28, he's two years older than me, he made me feel like a bossy aunt demanding to know how school's going. I asked him if he felt at all daunted by playing Shakespeare. "Yes," he whispered, looking round the room as if for help, "I did."

Shakespeare in Love is a bit of improvisation on a what-if idea: what if Shakespeare had writer's block during the writing of a comedy called "Romeo and Ethel, the Pirate's Daughter"? What if he suddenly falls in love with an unattainable woman? And what if this experience feeds into the play he's writing, to make it the tragedy of doomed love we all know?

It has the potential to be disastrous, heinous even. But Madden's energetic direction, a crackling, pacy script courtesy of Elizabethan aficionado Tom Stoppard, and several committed performances make it a fast, rude, passionate film with a distinctly Shakespearian structure: a bawdy romantic comedy of mistaken identities with a larger theme running just beneath the surface - the processes of and pressures on artistic creation.

It's only when I tell him that I liked the film that he's a bit more forthcoming: "Shakespeare's sacred ground, isn't he? An academic's hero. Everyone's hero. So there was a feeling of trepidation in taking him on, yes." He admits to being "a bit shocked that Shakespeare was being approached in this Disney fashion," but is full of nothing less than elegiac praise for Stoppard. "The script isn't an in-depth look at Shakespeare and whether he really was the genius behind the pen. It's Stoppard's play on wit. If you want to make idols accessible - which I think Shakespeare should be - then you have to bring a human touch, make it self-effacing and warm. And that's what Tom does. What he's saying is that 400 years ago isn't that long and the parallels between the Elizabethan age - and its competitive nature in terms of the different theatres - is probably very similar to London or LA. I love the world Stoppard's invented. I think he's come close to how people really did exist - I imagine it must be like MPs going to Soho peepshows, all those people at court crossing to Cheapside to watch Romeo and Juliet."

He is surprisingly pragmatic and self-effacing in his approach to acting, referring to it as "a job I've been doing for seven years." Preparation for the role consisted of "visiting a library. Reading some plays was a real chance for me to catch up on the horrendous schooling I had". But Fiennes doesn't really believe in preparation: "there's a fine marriage between the information you collect and the world of the script. Often you can do too much. You have to be prepared to give yourself and your ideas up to the writer's imagination."

Inevitably, I found it difficult not to mention the R-word. I ask him, tentatively, if he's found it difficult having a famous actor as a brother, and it's the only time in our 45 minutes that he displays any glimmer of a temperament beyond co-operative shyness. "That is one of the few journalistic angles which is the obvious one," he says rather tersely. Then he collects himself: "and I can't blame them. I mean if I was a journalist, I'd do the same. But it hasn't. Really. Not at all. Ralph and I don't sit about discussing acting, you know."

He is the youngest in a family of seven. His father, Mark, is a photographer; his mother, Jini - who died in 1993 - was a writer and artist. The children had a peripatetic upbringing, moving about between London, the West Country and Ireland. "I wish I could paint a picture for you of a kind of bohemian idyll, but it wasn't. It was a normal, messy, smelly, noisy environment." He is unmistakably proud of his siblings: "we're all more or less involved with movies. One is a director Martha, who directed Ralph in Eugene Onegin , one a composer Magnus , Sophie's a producer, but Jacob, my twin, is a gamekeeper."

Joseph left school at 16, initially to study art, but after a year he moved to London, got work backstage at the National Theatre (where Ralph was already appearing), and began acting in various youth theatres, gaining himself an agent and then a place at Guildhall School of Music and Drama. From there, he progressed to stints with the RSC and in various West End productions. "Working backstage as a teenager made me realise there's not much glamour in this profession - just lots of hard work. That's a good thing to learn early on."

Next in the ascendant career of Joseph Fiennes is a part in the screen adaptation of James Hawes's novel Rancid Aluminium. And this time, it's goodbye to lovelorn swains: "I play an Irish lawyer who tries to save his best friend's business from the taxman. Halfway through you realise he's not the sort of lawyer you want handling your books." Does he like playing the baddie? "Oh yes," he says, then thinks for a moment before adding, "although I see him as completely sane".

After that Fiennes is heading for the first time to Hollywood for a Paul Schrader movie and another bad guy: "I'm very excited about it. It's a piece he wrote in the 1970s which he hasn't updated so it's very much of its time. My character is an all-American kid who changes identity and comes back to wreak revenge and to get the girl. But he's all scarred and she obviously will find him quite horrific. There's lots of prosthetics and nastiness - a revenge tragedy."

Fiennes is adamant that the lure of Hollywood won't keep him in America. He will come back to London - and the theatre. "I like language, and in film language is diluted by the visuals and the music. Theatre is what I was trained for." Is he tempted now to take on a Shakespearian role? He shuddered visibly. "No. I couldn't bear to walk onstage knowing that half the audience knew the play better than me and were mumbling it along with me. I'd rather do a production no one's ever seen. I think Shakespeare should be given a rest."