INTERVIEW: After the death of his brother, River, Joaquin Phoenix is understandably reluctant to rake over his family's past. By Ryan Gilbey

Joaquin Phoenix is now 25 years old, two years older than his brother River was when he died of a drugs overdose. He is also at roughly the same stage in his career as River had been: the point where an actor stops being a curiosity and starts getting his name above the title. River had begun to punctuate his maverick career choices with parts in varnished Hollywood blockbusters, and this month Joaquin completes the same journey with a leading role in Ridley Scott's Roman epic Gladiator.

There is always a ripple of panic when an actor prized for his uniqueness signs up for a movie where the catering budget alone could wipe out the national debt, so it's reassuring to report that Joaquin's integrity should increase along with his fee after Gladiator.

He plays Commodus, a brat whose immorality knows no bounds - he lusts after his sister and, when he is passed over for Emperor in favour of General Maximus (Russell Crowe), he kills his own father (well, what can you expect if you name your son after a toilet?). The glory of Joaquin's performance is that it circumnavigates all the clichés of cinematic villainy: he mines Commodus's psychological frailty to create a moving portrait of a despised son who turns hateful in his scramble for love.

If you were looking for signs that Joaquin had made it, that he was on the verge of ceasing to be just "River's kid brother", then you could have found it in Rome last week, where the shy, mumbling actor was receiving journalists in less-than-intimate mobs of eight or nine, the true mark of stardom. (Still, on the plus side, no one asked him how to pronounce his name - it's "Wah-keen" in case you were wondering.)

The first surprise when you meet him is how small he seems compared to the buffalo-like colossus swanning around in a toga in Gladiator, and how much he really does resemble those elegant portraits from the Prada advertising campaign (the one where this committed vegan refused to model below the waist since it would have meant endorsing the company's leather shoes). It transpires that the increased waist-line and the additional number of chins which he sports in Gladiator was all his idea.

"I put on 15lb," he explains, dragging on a cigarette to satisfy an addiction which he talks of relinquishing on an hourly basis. "When I take a part, I become obsessed with physicality. I wanted to be very fit, very muscular for Gladiator, so I started working out. But then I realised that Commodus had achieved what he wanted and become very lazy, so I wanted to show the decay and decadence. I started pounding pasta and stopped working out. Apparently Ridley was watching the dailies and he suddenly shouted: 'F***! He's fat!'

"I really didn't know what I'd got myself into until the first day. I had looked at my scenes and most of them were quite intimate. I would read: 'Scene 127: Commodus walks toward palace.' I'd think: 'OK, that'll take an hour, couple of wide shots, whatever.' Then I'd get there and see 150 troops, 200ft statues, three cameras, a crane, some camels. Which is when I thought: 'Oh, so that's how you make an epic.'"

Joaquin's knack for physically transforming himself for each part indicates the malleability of a true character actor who will never become anchored by the affections of teen magazines. River's popularity began in the pages of Jackie and Just Seventeen, but Joaquin's more wolfish, wicked face makes him striking rather than pretty: the eyebrows like approaching storm-clouds, the vivid eyes that pierce and probe, the scar above his mouth - which he was born with but which is often wrongly assumed to be the result of a hare lip operation.

He knows how to manipulate his physical quirks: in Gus Van Sant's black comedy To Die For, in which he played a loafer seduced by Nicole Kidman into murdering her husband, he had a loose-jawed, almost Neanderthal quality - you could practically hear his knuckles scraping along the ground. In Oliver Stone's U-Turn, he resembled a plastic arcade cowboy, with his square head and galleon-shaped quiff. When he dyed his hair blue and climbed into vinyl trousers for 8mm, he looked like something that Nicolas Cage's private investigator had found under a particularly unsavoury rock.

His affection for society's eccentrics and underdogs is not waning. In the forthcoming thriller The Yards, screening in competition at Cannes this year, he plays a rail-yard worker who sabotages competitors, while in the Marquis De Sade drama Quills, he stars as a corrupted priest, and gets to play what have been described as "tough" sex scenes with Kate Winslet.

With such a bright future, he could be forgiven for his reluctance to rake over the past when a French journalist presses him on his upbringing: "You were a child actor," the reporter begins. "How should we look at a family where that happens?" "Don't," he snaps back, quick and bright as lightning. "Just look at your own family and have fun." "But how did you come into acting?" persists the journalist. "Just got an agent and... " "No," the man interrupts. "You don't start with having an agent."

"Well, you seem to know," Joaquin laughs, incredulously. "Why don't you tell me what I did with my life? You ask me and then you say 'no'?!" "But to get an agent," continues the man, "you must already have done something." "You're the authority," replies Joaquin, "you tell me." He's still giggling. "Look, you go to an agent, you see if they're willing to take you. You do auditions, commercials... but it's really boring to talk about."

For anyone who followed River's career, it's an old story: hippy parents with five children flee a religious cult when its practises are revealed to be unorthodox and hotfoot it across America, promoting the musical talents of two of the brood (River and Rain) until a Paramount casting agent shows an interest in the youngsters.

Joaquin, who changed his name to Leaf at the age of four, and switched it back when he was 18, chalked up childhood appearances in films such as Parenthood and Spacecamp, but jacked in the business in 1990 for five years to go travelling with his father. The choice was ostensibly due to the quality of the scripts he was getting, but his return must have been partly delayed by the media's handling of his brother's death - Joaquin's own desperate call to the emergency services was repeatedly broadcast on US TV, while photographs of River in his coffin were quickly circulated.

He maintains that the industry's lack of imagination is still prevalent. "My first offer after To Die For was a TV movie in which a teacher seduces her student and gets him to kill her husband. I kid you not." As for the media, he is as sweet-natured as anyone could expect him to be. That crotchety French journalist even gets a playful hug. "I don't want you to leave this room with bad feelings about me," jokes Joaquin, trailing a wisp of cigarette smoke behind him as he makes what might be called an exit, but which anyone could tell you is more like an escape.