His elder brother Ralph may be a tough act to follow, but Joseph Fiennes is already in hot pursuit with his powerful portrayal of Jesus in Dennis Potter's 'Son of Man.' Report by Sarah Standing

Unlike lightning in the natural world, theatrical success within a family does occasionally strike twice. Just as 1994's undisputed hot ticket was Ralph Fiennes giving his Hamlet in Hackney, this year it is his younger brother, Joseph, who dominates the Barbican stage with his powerful and moving portrayal of Christ in Dennis Potter's Son of Man.

It is an extraordinary performance on many levels. No matter how feeble your faith may be, Joseph has managed to turn Christ into a man that Everyman, including cynics and disbelievers, can identify with not only on a human level, but also on one that is totally plausible and timeless. When Son of Man was first shown on television in 1969, Mary Whitehouse attempted to prosecute Potter for blasphemy, an act that seems ludicrous and misplaced in today's social climate. Doubt, anger and often despair are at the heart of any religious fervor and these are the emotions Potter gives Fiennes to play with.

It is a daunting part for any actor, especially one whose biography is so short that when the RSC faxed me a copy they apologised for its brevity, adding the footnote, 'He is very young!' Since leaving drama school, Joseph has appeared in just three West End productions: Woman in Black, A Month in the Country with Helen Mirren and A View From the Bridge. He's done The Vacillations of Poppy Carew for television and has recently finished his first film for Bertolucci. He is still only 25 and so far it has been plain sailing.

Director Bill Bryden, who cast him opposite Mirren and subsequently as Christ, says, 'Joe's quality is his seriousness - his lack of knowingness. He's easy to work with because of his intelligence. He has innate good taste and all the best attributes that come with being an ambitious young actor without any of the negative bits. He's incredibly popular with the cast. During the rehearsal period for Son of Man, when we finally got round to doing his big speech when he delivers the Sermon on the Mount, the entire cast stood up and applauded him. That's rare.'

Perhaps it is his extreme youth that makes his performance both haunting and thrilling. An older actor would have brought too much emotional baggage to the part. Joseph says he found it daunting at first, but once he discovered Potter had written the part not as a divine creature, but more as a man plagued by the enormity of his destiny, something clicked. 'I had more of a rural upbringing than a religious one, so for me Jesus has to be plausible. He's constantly questioning his role.'

Indeed, Fiennes implores, 'Is it me? Is it me?' at the beginning of the play, which is initially set in a carpenter's shop. The critics seem fairly unanimous in their answer, as do the audience crammed nightly into the Pit Theatre. He moves them to tears and led the Evening Standard critic to declare Son of Man one of the most violent pieces of drama he has seen on stage. There is no blood or gore at the Crucifixion, so the violence isn't graphic; it is the intensity of Fiennes's suffering that makes it so strong.

It is probably just as well his next part, again for the RSC, is that of a murderer in Les Enfants du Paradis, giving him the opportunity to go from saint to sinner in the space of six months and thus avoid being type-cast as the nice guy. In real life, though, he is just that. There's no Cain and Abel conflict of thespian interests between him and brother Ralph. They adore and admire on another. The discrepancy between their ages, coupled with their DNA make-up, is luckily different enough not to invite comparisons. Joe has been happily ensconced with actress Sara Griffiths for the past four years. He is in the middle of moving house and the only perceivable blight on his otherwise rosy horizon is the fact that it has become too cold for him to ride his Vespa to work.

We meet in a photographer's studio in the City. His experience of being interviewed is limited, but he spent the morning in a school at the top of his road being put through his paces in a question-and-answer session with the school's drama group. (He agreed to do it because he has locked himself out of his flat so many times and had to climb through a neighbour's window - and the neighbour's child asked him.) You get the impression Joseph doesn't feel comfortable talking about himself excessively in any circumstances. He is much too well brought up to be a 'luvvie' and I doubt the would ever 'go Hollywood' unless he was certain of a return ticket.

Bryden has a theory about the new generation of actors. 'Joe's contemporaries, such as Ian Glen, Rufus Sewell and Toby Stephens, are very clean-cut without being boring. I don't think we're going to catch these guys out because they're proper actors - to them the quality of the work they do is everything. Of course, they want room service and limousines like everyone else, but the text has got to be worthy of it before they'll accept the job. They're not interested in being the next Hugh Grant.'

A kind of myth has evolved around the Fiennes family, the general perception being that they are this well heeled, vaguely eccentric Catholic clan that has inexplicably produced two rogues and vagabonds. When you explore their upbringing it all makes sense. 'My childhood was mad, bad and wonderful. It was mad because we moved house so many times. I always tend to exaggerate about this, but I think the final count was 14, so I grew up in various parts of the West Country, Ireland and London. It was a wonderful up-bringing with lots of dogs, outdoors and adventures. There were masses of books everywhere and wonderful paintings and hardly any television. Everything you could want as a child.'

Joseph's father, Mark, is a photographer whose wanderlust for new locations was fuelled by the need to make financial ends meet. His mother, Jennifer (who died two years ago of breast cancer), was a novelist and painter who somehow managed to write six novels while bringing up six children of her own and a foster child with very little help. Their daughter, Martha, herself a film director, says, 'She believed totally in motherhood. We were her special project and she gave it 100 per cent. She instilled in us the importances of "putting your guts into it," no matter how small or insignificant the task was. When we lived in Ireland she got permission to teach us herself for a period, and everyone always thinks life must have been chaotic and hippyish, when in fact it was incredibly disciplined. She always treated us as individuals and found separate time for all of us, which was no mean feat. It worked because we adored and respected her and really cared about her opinion.'

According to Martha, Joe went through his difficult, rebellious stage when he was only six. 'It's a family joke because he was a member of a horrible gang at school called the Rough Gang, who were very naughty, and our mother was really worried. He snapped out of it by the time he was seven and never went through the normal teenage rebellion. He just turned overnight into this incredibly sensitive, mature little boy and then grew into a mature sensitive man.'

Joseph's sense of values is very down-to-earth. When he talks about his twin brother, Jake, who is a gamekeeper, he describes him admiringly as 'a man of the land.' Joseph instinctively goes for the cerebral and aesthetic pleasures of life and always carries a sketch book with him -- a legacy from the year he spent at art school. He says he's not a social animal, he doesn't really like all the stuff that goes with it.

Breast-beating is obviously anathema to him, as is talking about himself. It doesn't really matter, because it looks like everyone else is going to do it for him. The word is out, he has the elusive 'it' factor an actor needs. With his rather incongruous newly grown beard, he has that shy confidence and stillness that women find dead sexy.

He pulls on his woolly hat, cadges a Silk Cut, thanks me and the photographer and makes his way back towards the Barbican for the matinee. He may have his cross to bear, but by God, he bears it well.