Joaquin Phoenix once left his Joker co-stars stunned by walking off the film's set in the middle of a scene because he "just wasn't feeling it".

The Walk the Line actor has been earning rave reviews and early Oscar buzz for his performance as the iconic Batman villain, but writer/director Todd Phillips has revealed the dark nature of the role sometimes got too much for Phoenix and he would leave the set midway through shooting a scene.

The Hangover filmmaker told The New York Times his leading man "lost his composure on the set, sometimes to the bafflement" of his colleagues.

"In the middle of the scene, he'll just walk away and walk out," he explained. "And the poor other actor thinks it's them and it was never them - it was always him, and he just wasn't feeling it."

However, the 44-year-old would return to the set after taking a time out, and he never did so while performing with Robert De Niro, who plays late-night talk show host Murray Franklin, who Phoenix's character Arthur Fleck idolises.

Describing his co-star as a "consummate professional", De Niro added, "Joaquin was very intense in what he was doing, as it should be, as he should be... There's nothing to talk about, personally, on the side, 'Let's have coffee.' Let's just do the stuff."

In the profile piece about the actor, Phillips also admitted Phoenix disliked the explicit references to Batman, such as his character meeting a young Bruce Wayne, Batman's alter ego, and Bruce's father Thomas.

"He never liked saying the name Thomas Wayne," the moviemaker said. "It would have been easier for him if the movie was called 'Arthur' and had nothing to do with any of that stuff. But in the long run, I think he got it and appreciated it."

Joker, which won the Golden Lion for best film at the Venice Film Festival in Italy at the weekend, hits theatres from 2 October.