Joseph Fiennes is not entirely to be trusted. Several years ago, a tabloid journalist bullied him into revealing more about his personal life than he was happy with. So, when she went off to the loo, he calmly rewound her tape recorder to the beginning. A wonderfully subtle act of revenge-typical of Fiennes' laid-back style. "I can be a bit naughty," he admits.

When we meet for lunch at Nobu, the too-cool-for-words temple of sushi, I don't take my eyes off him. He's come from a hotel room where, all morning, he's been promoting the new DreamWorks cartoon, Sinbad: Legend of the Seven Seas (he's one of the stars drafted in to voice a character; others are played by Brad Pitt, Catherine Zeta-Jones and Michelle Pfeiffer) and my heart sinks at his opening gambit: "Doesn't Harvey [Weinstein, the Miramax boss and co-owner of Nobu] have a private room here we can use?" It turns out, he's just worried my tape recorder will be drowned out the by the celebrity brouhaha. "I've never even been here before," he shares. Then, later, witnessing my unstinted clumsiness with Asian cuisine - "Aha, you're chopsticks challenged" - he offers a masterclass.

But then Fiennes is an intriguing mix of contradictions: a full-blown film star who insists on doing risky, minimum-wage theatre; a deeply private individual who nevertheless hits the headlines for his trysts with models and actresses; an artist who eschews celebrity except when it helps him campaign for causes he believes in (he has just been to Angola for Christian Aid).

Like many people, I'd assumed Joseph was, well, a bit Ralph-lite. A less brooding, less than enigmatic version of his elder brother. And certainly earlier films such as Shakespeare in Love and Martha, Meet Frank, Daniel and Lawrence marketed Joe as a clean-cut pin-up for teenage girls. But in person, you send more complex intelligence. Not flashy (he talks openly about his dyslexia), he has a nicely ironic line when it comes to his own fame: "acting for me is more about psychology and communication than starring opposite Clint Eastwood." Of course, there's no denying the beauty-soulful eyes, fashionably jagged hairline, gym body - but that scar on his lower lip is a welcome imperfection.

Unkind critics mutter that Fiennes's Hollywood stock has dipped (and it's true that he's had a string of film duds including Rancid Aluminium and Killing Me Softly). But you sense he chooses to take risks and plump for projects that excite him. Doing what seemed mad by Hollywood standards, after the huge success of Shakespeare in Love, Fiennes appeared in a play at the Royal Court and then went backpacking around India. In fact, as he reveals, "I'd gone to India with Shekhar Kapur to promote Elizabeth, and went back a year later because I found it so engaging. It was seized on as this weird romantic Byronic fantasy of giving up the film industry for the bohemian lifestyle, but I do what everyone else does - I trip out and I spend a couple of weeks in foreign land."

Of his recent film choices, he shrugs, "It's alchemy, you can't get gold all the time. I just feel pretty fortunate that I'm at that stage where I can juggle different directors, different genres, based on what's on offer. And also I've chosen films that take me outside America."

He's amused by the way the media hypes - then discards - the latest Beautiful Young Thing. "Something pops and suddenly you're pigeonholed as heart-throb this and heart-throb that, it's all cyclical," he laughs. "Hollywood is very much wedded to youth and beauty. There used to be 40 magazines, and now there are 400 all about diets and yoga and liposuction, and there's this corruption. But coming from a theatre background, I see acting as a live-long endeavour. I think range is freedom. There's nothing more exciting than to see an actor grow and mature -from playing Romeo to Mercutio to Benedict to Hamlet to Lear - all those stages."

And he really can act. It is telling that for his return to classical theatre in 2001 - in Edward II at Sheffield Crucible Theatre - Fiennes chose to play a gay anti-hero in the provinces. "It's a ruthlessly modern, muscular, angrily intelligent piece," he enthuses of Marlowe's play. "And to bring that infront an audience, the majority of whom were aged between 16 and 24, and who may never have been to the theatre before, was such a challenge." Indeed, one critic was moved to declare, "Fiennes, with his long, sensual face and resonantly expressive voice, once again proves that he's a much sexier actor than his brother Ralph, who too often gives an impression of emotionally constipated prissiness." And he also has a gift for comedy - gaining great reviews for his performance as Berowne in Trevor Nunn's sparkling production of Love's Labour's Lost at the National Theatre last February.

For a man whose exes include the actress Catherine McCormack and supermodel Naomi Campbell, Fiennes is famously private. Though certainly not stand-offish. At the end of a Marie Claire Shoot sometime ago, his taxi failed to turn up and, just as the art director and editor was squabbling about who would drive him home, the make-up artist whisked him off in her MG -they ended up living together for several years. These days, he's resputed to be dating Australian actress Natalie Mendoza, but personal questions are gently deflected. "I think an actor has to have ownership of a certain part of their private life, if everything's exposed, you'll go nuts. You'll never feel you've lived according to your own code. You'll always be pleasing other people, and that's not right. I have no complaints. I can travel on the Tube...it's only when you do a bit of publicity that people take notice."

He's amused that everyone asks him about being a twin (Jacob is a gamekeeper). "I wish I could say we were telepathic, because journalists are so fascinated. We shared birthdays and space in the womb, so we are going to be close, but we're fraternal not identical twins."

When I interviewed his sister, the director Martha Fiennes, several years ago, she mentioned that, as children, Joseph and Ralph were the show offs in front of the camera ("look at me, look at me") while she and her sister Sophie (also a director) preferred to remain behind it. Joseph laughs, "It's interesting. My dad was a photographer and they were very much into photography and film from a really early stage. They just gravitated towards it."

Not that Fiennes takes the role of cinema lightly. For him, Sinbad is not just a state-of-the-art cartoon. "The reason I want to do this film is that I do believe animation is important. I was brought up on it. Everything from The Simpsons to the first black-and-white Mickey Mouse cartoon, what they explore, in a mythological context, is the childs ability to deal with big themes which they are going to have to embrace somewhere down the line. So animation is a great way for children to take on the responsibility of actions dealing with big, key traumatic things-love, death, betrayal - without the formalism of reality, if you like. So you have the wicked witch or the Child-catcher in Chitty Chitty Bang Bang. It's terrifying, but if we didn't meet them, we wouldn't know later how to deal with the bailiff or the tax collector or how to deal with being shouted at on the street or being back stabbed by a friend. And, "hey, listen," he enthuses, "The other thing is it's a girl's movie . It really is. To have a girl at the helm alongside Sinbad, that's DreamWorks. I think it's brilliant to get that balance and juggle those elements."

In Sinbad, Fiennes plays the heroic but slightly dull Prince Proteus who loses a girl to Pitt's muscular "Sin-Brad". The animation has based their remarkable drawings on each actor's physiognomy, and you can imagine DreamWorks going, "Get us the handsome one in tights from Shakespeare in Love."

But Fiennes is genuinely excited about Proteus. "He wants to be Sinbad in many ways and learn about the world outside the palace, but later on the has to honour his royal obligations and sacrifice that distinctive part of him, that burning desire to go on the high seas, to get a girl, to fight the senior monster. The film absolutely spoke to my boyish instincts. He's in touch, he's a good thinker, it's a political marriage, I guess, between him and Marina... Listen to me talking about an animation in such in-depth terms. It's like 'Joe, it's a cartoon!' But then you think, it's not cartoon, in the way these guys have worked for four years, and the level of craftsmanship that goes into it, and because of that I've given it is much thought as I would a feature film. Until I went to DreamWorks, I had no idea of the amount of drawing that goes into a single frame. I have massive respect for their work. A lot of artists from Russia, from Italy, from France, it's a great cross-cultural collection."

Typical of cartoon blockbusters, Fiennes had to voice he's role alone in a Los Angeles studio without the other actors, while the animators filmed his every gesture. "Somewhere, in someone's living-room, God knows, they're watching a tape of me fight imaginary sea monsters," he observes drolly.

Joseph Alberic Fiennes was born in Wiltshire in 1970. He and Jacob are the youngest children of photographer Mark Fiennes and writer/painter Jini Fiennes. The family later moved to West Cork in Ireland, where their parents made a living renovating old houses. Joseph estimates he changed school 14 times - a good training for the gypsy-actor existence.

At 16, Fiennes left school by college, but soon moved into Theatre, taking work at the National. After graduating from the Guildhall School of Music and Drama in 1993, he joined the RSC for two seasons, then, in 1996, won a cameo as a young gay man in Bernardo Bertolucci's Stealing Beauty. It was in 1998, when he played romantic leads in three British films, that he really broke through. The frothy romantic comedy Martha, Meet Frank, Daniel and Lawrence was followed by two major Elizabethan roles - Robert Dudley, opposite Cate Blanchett, in Kapur's Elizabeth, and then he starred as the eponymous bard in Shakespeare in Love. Fiennes says he had a ball making the film. "It was like a theatre company coming together as one unit. I remember Geoffrey Rush turning to me between takes, and saying, 'you know, this is going to be cracked. We are having too much fun.'"

In addition to Sinbad, Fiennes has three new films in production- a movie version of John Osborne's Reformation play, Luther, John Dahl's military thriller, The Great Raid; and Leo with Elizabeth Shue, Sam Shepherd and Dennis Hopper. He seems most proud of Leo. "It's such a beautiful, original tale from a new young first-time director called Mehdi Norowzian. I just wish we could get it released,"he sighs. "It's about a young boy who writes to a man who is in prison for murder. The boy comes from an abusive background, his mother's an alcoholic, he never knew his father, and through their letters they share a way out of their pasts. And eventually the man comes out of prison and visits the boy's house... and it transpires that the man is actually the boy grown up."

But then Fiennes is a thinker. He is genuinely regretful that critics didn't comment on the global echoes in Trevor Nunn's First-World-War-themed Love's Labour's Lost (ironic really when they have praised Nunn's successor, Nick Hytner's production of Henry V for its Iraq parallels). "Love's Labour's Lost was bang-on," Fiennes enthuses, "but no one picked up on that. I remember when we were rehearsing it saying to Trevor, 'God, the impending doom of what's going to happen is going to coincide exactly with the play.' And Trevor gave a wry smile. So he is it right on the head, because the play is all about using the metaphors of battle and War-cutting, rushing, shooting, killing - as an analogy for love."

Straight after the play, Fiennes raised his own head above the parapet. After reading a draft of Christian Aid report detailing how oil fuels poverty, civil war and corruption in developing countries, he agreed to go to Angola as an independent witness. Launching the report, Fuelling Poverty, after his visit, he admitted it had been a life-changing experience. "Until I sat with a girl with full-blown-Aids-infected by rape or a husband returning from a mining brothel, and left outcast, stigmatised and desperate about a children's future - I didn't really understand this paradox is plenty. But I also see the pride and dignity, the strength and the hustle to survive against all the odds."

Christian Aid's Wendy Bailey says she chose Fiennes because "he reaches out beyond his own sphere and cares about what is happening in the world. It took an awful lot of time. The trip itself was hot, uncomfortable, and unglamorous, sometimes dangerous -there are landmines all over Angola. It's not a superficial freebie, it's a commitment. When George Bush went to West Africa, he chose not to go to Angola, because it's too dangerous. Well, Joseph Fiennes went."

Best of all, Fiennes sees the bigger picture. "Now, when there is daily news about how the oil wealth by Iraq is to be carved out, it is critical of the world finally get it right on oil. I think we're at a really pivotal point where we won't be kept in the dark and the fact that millions are walking the streets in protest, and are asking more questions, I think signals the time when people truly want to be involved in these discussions that are affecting all of us. And part of the Angola trip was, I guess, me saying, 'I would like to know about oil in developing countries, such as I Iraq, which is going to be carved out. Who gets the oil? Why did France not want to the war to go ahead, because they had all these deals the same as Russia?' America is now close to agreeing the carved up of the minerals in other People's countries. And, you know, we are taxpayers and a proportion of our tax money goes towards the expiration, sanctioned by the World Bank, of how to export oil from developing countries.

"And Christian Aid was about giving me the chance to witness the incredible work it does exposing the hot-spots of suffering in various parts of the world. I could see and hear myself, and street level, what's going on, how the majority live, how they are affected in an economy that is dependent on the West's addiction to oil, and the way it turns a blind eye to Angola situation-which is that people there don't have been water, they don't have schools, they don't have hospitals. Ninety percent of the country's revenue which is close to $5bn a year is brought in by oil and none of it goes towards the central infrastructure."

Of course, film stars aren't supposed to be polemical. And when I ask how his role with Christian Aid sits with, say, a DreamWorks junket, Fiennes is very careful to distinguish between party politics and environmental aid. But he offers a forceful code to our interview. "I think we all have to be wide awake with the way Iraq is going to be dealt with - how the oil serves the people of that country. This is about issues that affect us all. Yes, I want to be involved in the discussion, and I don't want to be lied to. And cynical, I feel I'm being lied to, and that I'm really being caught out. I want to be in the light. I'm not happy in the dark."

'Sinbad: Legend of the Seven seaS' goes on general release this Friday.