"If you have somebody who has that level of emotional disturbance, I think that they can find fuel anywhere," Phoenix said.
Warner said its job is to provoke difficult conversations.
"Joker" has been the subject of much debate since it premiered late last month at the Venice International Film Festival, where it won the top prize, according to The Associated Press. It's also expected to be a major awards contender.
The film is a character study of how Arthur Fleck, a mentally ill aspiring stand-up comedian and clown-for-hire, becomes the classic Batman nemesis.
The Aurora theater was remodeled and renamed after the 2012 shooting that also wounded 58 people.was convicted of murder and sentenced to life in prison without parole.