Nerves troubled Joaquin Phoenix at the start of the biggest role of his film career. The young actor simply wasn't convinced he would be credible as the demented Roman Emperor Commodus.

Both Ridley Scott and Russell Crowe knew he was made of the right stuff but were worried he wouldn’t relax into the role. So they used an unorthodox method to loosen him up, thought up by Richard Harris, who plays the aged Emperor Marcus Aurelis. “Harris said ‘Let’s get him drunk’,” says a laughing Russell. So one night after filming they all sat down, had a few beers and told Joaquin that he was rated as a quality actor. “Joaquin realised, ‘I’m an actor, so I can relax a little bit more’,” said Russell.

Joaquin smiles broadly as he recalls the process of preparing for the role of Commodus, a ruler who murdered to gain an empire and will commit heinous acts to hold on to it. “Initially, I was not sure where to start,” he says. “He was a great character in the script, and there are a number of indications about his history and his youth, such as the clear sign he was a neglected child. A lot of historians have gone on about his more malicious nature. But one historian, Gibbon, describes him as having a weaker disposition, which fell in with my interpretation.” Joaquin reckons he had an opportunity right at the start of Gladiator to illustrate the weaknesses of Commoduc, when the heir to the Roman Empire arrives at the battle against the barbarians after the fighting has ended.

“I thought it was key to show at the beginning of the film how out of place he felt. He sees how Marcus Aurelius and Maximus command the respect of the troops, something he feels incapable of. Then it was just a case of getting into a paranoid state of mind as the character progresses through the film.

The fabulous costumes also helped. “Going through the first rehearsal dressed in my jeans, I was thinking I can’t do this!Then I put on the layers of armour and felt right. Costume and make-up make the difference, especially as I’m obsessed with the physicality of a character.

"At the beginning I permed my hair twice to look like the young, scraggy prince-in-waiting. Then, after he became emperor, I cut my hair and stopped going to the gym. I wanted to put on a few pounds and display some of the lazy decadence that would come from being an emperor.”

While Joaquin concedes that the transformation from Hollywood actor to Roman monster was never easy, he adds that every film role he has undertaken has been painful. “In the best sense,” he says with a chuckle.“I dread the day I go to work and think I’ve got this thing down and I can just say my lines.

“In this film, the amount of exploration we did for my character seemed endless. Every day posed a new challenge.With every scene I discovered something new, a different part of his personality. That’s ideal as an actor because it’s exactly what you look for. Otherwise it could get dull.” He was excited as a kid on seeing the sumptuous sets for the first time. “I couldn’t believe it when I saw the German camp, with the horses and extras and everything.”

He laughs when asked if he had seen epics like Spatacus or Ben Hur before starting filming. “I’m so embarassed but I never saw any of them,” he says. “Two weeks ago I caught the last 20 minutes of Anthony and Cleopatra on TV.

“But ironically, I started to get interested in the genre just before I got this part, and I intended to buy them all. Then I landed the role and decided it would not be a good idea.  I thought it would be best to have a fresh approach.”