Five years after his starmaking turn in Gladiator, Joaquin Phoenix finds the perfect role for his off-kilter charisma as Johnny Cash in Walk the Line. Donald Clarke talks with the supposedly touchy actor about portraying a legend, facial scars - and life as Leaf.

SOME days before I fly to London to interview Joaquin Phoenix, his people pass on a message that there are to be no personal questions. (Happily, this happens less often than you might imagine.) Stick to the actor's Golden Globe-winning turn as Johnny Cash in the upcoming Walk the Line. Talk all you like about his breakthrough performance in To Die For. Muse upon his malevolence in Gladiator. But stay away from the several complex traumas in his past. Such is the implication.

Leafing through earlier interviews, I come across suggestions that he may be prone to taking easy offence. Oddly, the accounts of walkouts and huffy fits never seem to be based on first-hand evidence. It is said that . . . He is rumoured to be . . . Others have found . . . Still, none of this helps lighten my mood.

Phoenix's particular caution about this round of interviews may stem from the fact that the plot of Walk the Line allows journalists an entry into the grimmest event in his life. Like Ray, last year's big music biopic, this powerful film suggests that its hero never quite recovered from the death of a sibling. Johnny Cash's brother Jack was, according to the film, killed in a sawmill accident while the future country singer was off fishing.

Several interviewers have tried to draw parallels between Cash's early trauma and the death by overdose of River Phoenix, Joaquin's senior by four years, in 1993. It is, I suppose, understandable that Joaquin might get a little testy about such a sensitive matter.

With all this in mind, I have donned my softest kid gloves and, primed for recalcitrance, have come armed with many bland questions in the "What was it like working with . . . " line.

As it happens, Joaquin Phoenix comes across as the most easygoing fellow you could ever hope to meet. Shorter than expected and a little less stocky, he puffs cigarettes throughout the interview and deposits the butts in a glass in front of him. He laughs frequently and - as I realise later after daring a few vaguely intimate inquiries - does not seem unduly prickly about discussing his home life.

"There is a big difference between American interviews and European ones," he says. "Yes there is this big tabloid thing here - and nothing against America - but here the journalists do seem a little more interested in the material."

If he wants to talk about the material, we'll talk about the material. Walk the Line is fine piece of work. Though the picture does suffer from some of the usual biopic infelicities (look, that is not Elvis Presley), Phoenix and Reese Witherspoon, who plays June Carter Cash, shun hollow impersonation to construct rich, nuanced characters. The script follows Cash from humble beginnings in Arkansas, through his time in the army, to early success and the inevitable descent into drink'n'drugs hell. It closes with redemption as June, a member of the legendary Carter Family singing troupe, helps him pull himself together.

Any number of challenges faced Phoenix, who sings all his own songs. Was he always confident that he could carry it off?

"I said yes without really thinking about what was involved," he admits. "I knew it was going to be very challenging and I wanted that challenge. Then suddenly I have said yes, I have a plane ticket and I think: am I up to this? There wasn't one individual thing that was the most challenging. It all felt daunting at the time. But so much of the singing is connected with his speaking voice. When that came together, everything else did."

Unlike June Carter's voice, which could dip from a warble to a holler in the space of a bar, Johnny's was, perhaps, not quite so technically impressive. It was all character. Wasn't it?

"You would be surprised," Phoenix says. "John had a great deal more range than you might think. A song like Walk the Line is a very difficult song. Each verse is in a different key. I mean, he wasn't Freddie Mercury, but it was still very trying to get right."

Coincidentally, Phoenix had been invited to dinner with Cash some time before director James Mangold suggested he appear in Walk the Line. The Man in Black was a great fan of Gladiator and was eager to meet the guy who played the evil Commodus. We think of Cash as being a slightly fearsome fellow. I imagine he must have had an intimidating presence.

"Not at all," Phoenix says. "I did have all these expectations about what he would be like. He certainly was a big guy and he emanated a kind of strength. But he was so warm and down to earth that you quickly felt you were with an old friend."

Jamie Foxx told me that, while researching his role in Ray, he was reluctant to spend too much time with Ray Charles, because he feared that the older star's tics might reappear when he was playing the singer as a young man. Walk the Line begins and ends with Cash's legendary 1968 performance in Folsom City prison. That Johnny Cash was a very different man to the fellow Phoenix dined with over 30 years later.

"I didn't do any research into John's life after 1968," he says. "But what I took from that meeting was to do with the dynamic of John's life with June. It was important to me to see that their love was not just a movie thing. They were two people who really understood one another. And I got a sense of the contradiction that is in Cash. There are these two people: the one June called John and the one she called Cash."

So, was Cash the public persona and John the real man? "In a way. John did cultivate the rebel image. He liked guns. He had a fascination with the darker side of life. But John relished both sides of life. Before I left he quoted some of my dialogue from Gladiator at me - these horrible, nasty lines - and this was a guy who a few minutes before had been looking sweetly in his wife's eyes and telling her he loved her."

Sadly, Cash fell ill before the two men had a chance to meet again and subsequently died in 2003. But, if some reports are to be believed, Phoenix remained possessed by Cash's spirit for some months after shooting ended. In April 2005 the actor checked himself into rehab for alcohol abuse. With no real evidence to support the claim, many pundits assumed that inhabiting Cash's character - indulging in mad, method binges, perhaps - had exacerbated Phoenix's problem. I think we are getting on well enough to tackle this issue.

"Well, it sounds very dramatic: 'in rehab'. But actually it was more like a country club," he laughs. "Everyone has different standards. Most people that know me said you are not an everyday drinker. But I was inspired by John's sobriety. John had reached a point that I can't really fathom with addiction, whereas mine was more of a preventative measure."

Having got that out of the way successfully, I consider raising the sensitive issue of Phoenix's wildly unconventional upbringing. The actor's father and mother met on the hippie trail in the 1960s. Sometime after the birth of River and his sister Rain - man, those names! - the couple became involved in a deeply sinister religious cult called The Children of God. Young Joaquin, his family having been sent forth into the world to spread the faith, was born in Puerto Rico in 1974.

It soon became clear that The Children of God's quaint name could not have been more deceptive. In the year of River's birth the church's leader, David Berg, introduced Flirty Fishing, which encouraged female members to physically express God's love by engaging in sexual activity with potential converts. Berg became increasingly eccentric and materialistic and, eventually, the Phoenixes got sense and stowed away on a cargo ship to Florida.

Actually, I don't think I can bring myself to ask about all this.

Anyway, once back in America, the family became drawn into show business. Joaquin, jealous that his siblings all had more ridiculous names than he (sister Liberty and Summer followed), demanded to be known henceforth as Leaf Phoenix. Under that moniker, he acted on TV and secured a role in the successful 1989 comedy Parenthood.

But, for reasons he has never satisfactorily clarified, Phoenix didn't seriously pursue a career as a teen actor. The next time we noticed him - Joaquin again - he was being exploited by Nicole Kidman's ambitious weatherwoman in 1995's To Die For.

"Well it was a weird thing. I was 15. I had done this movie, Parenthood, and that brought me all these scripts, but they were no good. I thought: if all I am offered is these crap teen movies, then I would rather just not act. Then I just got the urge again. I can't exactly say why. But there is this incredible feeling of discovery. It's just there. This challenge: how do you make something authentic?"

Phoenix made a notable impression in To Die For, but it was another five or six films before he really asserted himself in Gladiator. Oozing charisma and a slightly uneasy sexuality, Phoenix doesn't look much like any other leading man around. His most distinctive feature is that diagonal scar on his upper lip - a birth defect and not, he insists, a corrected hare lip. Working in an industry where the slightest hint of facial asymmetry can render you as hard to cast as the Elephant Man, Phoenix is to be praised for not having this mild irregularity seen to.

"Nobody ever did suggest having it fixed," he says with no apparent embarrassment. "But I don't think anybody expected me to achieve what I have. They all thought oh, he can be a character actor and pop on the screen for 20 minutes. But he won't be able to carry a film. I do remember an agent years ago saying: 'You need to go to the gym.' And I said 'well, OK, if that's what the character does, then OK'. It's ridiculous. You see a film set in the 1800s and the guys all have these sixpacks."

So how does his own sixpack look on screen? Joaquin has no idea. Bizarrely, the young actor claims never to have seen one of his films. I can understand why he hasn't felt the need to watch Joel Schumacher's 8MM, even if he is the best thing in it. But surely he'd like to see Gladiator or Walk the Line?

"No. There is a part of me that is curious. But it is too much of a danger. As long as I want to continue acting I don't think that I will benefit from watching my movies. I think that breeds a self-awareness that is a tar pit for an actor. If you are playing a character who has that self-awareness, then that makes sense.

"You go to a dinner party and you moderate your behaviour in that self-aware way, but you see people in their cars and they are picking their noses and so on. That's what you want as an actor."

Walk the Line opens on February 3rd