The movie is being developed by Warner Bros. and DC Entertainment as part of a new group of films that will exist outside of the continuity of its shared universe, which includes Wonder Woman, Justice League, Batman v. Superman and Man of Steel. Joker is already a part of the existing universe, with Jared Leto starring as the classic villain in 2016's Suicide Squad.

This isn't the first time Phoenix has entertained the idea of a superhero movie.

The three-time Oscar nominee was offered the role of DC villain Lex Luthor in Batman v. Superman, and was in talks to play Doctor Strange for Marvel. Ultimately, he passed on both projects, meaning that his participation in Phillips' Joker movie is not a sure thing.

In September, The Hollywood Reporter broke the news that Warner Bros. was courting Leonardo DiCaprio for the Joker movie.

Phoenix will next be seen in Gus Van Sant's Don't Worry, He Won't Get Far on Foot, based on the memoir of late cartoonist John Callahan. The pic recently premiered at the Sundance Film Festival. The actor is also set for the Annapurna Western The Sisters Brothers.