It's the end of a long afternoon of interviews and Joaquin Phoenix is not looking good. His hair is rumpled. His suit is rumpled. His shoulders are preternaturally droopy. He couldn't look worse if he'd just been pounded around the Colosseum by a strapping Russell Crowe.

Yet according to the stories surrounding the making of the epic Gladiator, director Ridley Scott pursued this wretched specimen for months, cajoling him into playing the haughty, ruthless Emperor Commodus. "That's a lie; I don't know where anyone got that from," mumbles Phoenix to his cigarette. "It's ridiculous, because I actually had to do the convincing. Why wouldn't I want to do this movie?"

Why? Because with the highly forgettable GI Jane, 1492: Conquest of Paradise and White Squall, Ridley Scott was showing signs of having lost his once-magnificent touch. Yet here he was, dusting off the kind of blood-and-sandal epic that had been put out of its misery 35 years earlier and left to rest in merciful peace ever since. Phoenix, however, makes it clear that he had no preconceptions on that score.

"I never saw any of the films from the genre at all," he admits without the least embarrassment. It wasn't the Spartacus tunics 'n' gore that lured him to the project. Nor was it the fact that it was to be a $100 million widescreen extravaganza, with computer-generated Blade-Roma settings, or the thrill of recreating history. The biggest incentive, says Phoenix, as he scribbles tortured doodles on the production notes in front of him, was the psychology.

"In the script, it was quite a humanistic story. I saw it wasn't just gonna be people in togas with swords, that they were really interested in the human element, so that was why I thought I wanted to do it."

He does it, in fact, with a power that takes your breath away. Here's a pampered child, who suffocates the father who doesn't love him and tries to seduce the sister who does, who callously threatens the life of his young nephew and wreaks bloody vengeance on his late father's favourites; who schemes, lies, abuses his power and plays very dirty. And yet there's something about him that niggles at your sympathy: as he gets more quietly maniacal, more diabolically determined, you're torn between wanting to hit him and hug him.

"Oftentimes with the more villainous characters, especially in a big-budget film, you never have the opportunity to see what makes them tick, or if you do see it, it's something kind of ridiculous," he says. "I just thought there was more, something more ambiguous and rich and three-dimensional to this character."

"I think it's the suggestion of how one is raised, what one's experience is if you don't have the kind of fundamental love that each child desires, and have a parent that can instil those morals in that child."

Phoenix shuffles uncomfortably in his seat. At the age of 19, this son of penniless flower-power parents, brought up in a wacko religious community before being pushed on to the screen, watched his older brother River OD on a Hollywood pavement after a meteoric rise to cinematic fame.

He speaks faster and quieter. "In that situation I think you're dealing with something that's already on edge and if they then achieve a great sense of power it could certainly send someone over."

Phoenix has said that he has come to terms with - though will never understand - his brother's tragic end, and he claims to find a catharsis in his role as the Emperor Commodus. "In some ways it can be therapeutic. I think when you go through a really intense scene you just feel like such a sense of contentment and calm that kind of washes over you. Ten hours and you're shooting the scene over and over again, crying or screaming or whatever, it's so intense. You know we all feel better after a good cry." His gleeful smile has returned. "Try it for 10 hours."

If intense, psychological drama has been his mainstay (in November he'll return to cinemas as the priest who battles to save the soul of the dying Marquis de Sade - Geoffrey Rush - in Quills, a film which, he says, throbs with dark humour), Phoenix swears that he's a light-hearted kind of guy.

"I grew up watching the Three Stooges and Woody Allen. But you know most of the comedies out there, I don't think they really work. If one of those comes across I certainly would want to do it. I enjoy humour more than anything, I don't really sit around banging my head and crying all the time."

Not even, it seems, when he's fully immersed in the tortured spirit of a psychopathic Roman emperor