The BBFC has also released guidance on what the film contains in order to help fans work out whether it is appropriate for them to see.
The BBFC has said: “Joker is a thriller in which an aspiring comedian becomes increasingly unstable after funding for his psychiatric support is curtailed.”
In terms of violence, the board has said the film contains: “scenes of strong violence include stabbings and shootings, with accompanying bloody injury detail.”
According to the British Board of Film Classification (BBFC), Joker is a 15, meaning only those aged 15 and older are allowed to see it.
Children younger than 15 are not permitted to watch the film, even if they are accompanied by an adult.
Speaking to USA Today, Phoenix said of portraying such an unstable character: “It’s shaky ground as an actor. I enjoy not knowing precisely what a character may do, but you want to have like a couple moments that you feel solid about.
“And that never really happened. We just became very comfortable with not knowing.”
On how an audience will perceive Joker, he added: “It presents a bunch of difficult scenarios and questions, and it challenges the audience to experience it however they want.
“Everyone I talk to seems to have a different reading: It could be interpreted in a number of different ways and so it’s almost interactive. I really love that.”