The actor has been earning rave reviews and early Oscar buzz for his performance as the Batman villain, and the film's writer and director Todd Phillips reveals playing the comic book menace didn't come easy for his leading man.
Phillips tells the New York Times, Phoenix "lost his composure on the set, sometimes to the bafflement of his co-stars."
He adds, "In the middle of the scene, he'll just walk away and walk out, 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..."
One co-star who appreciated what Joaquin was going through was Robert De Niro, who adds, "He was very intense in what he was doing, as it should be, as he should be."
"Joker", which was named Best Film at the Venice Film Festival in Italy at the weekend (September 07), hits theatres on 4 October.