The Gladiator star, 43, is Phillips' top choice for the role in his DC movie, which will be produced by Martin Scorsese.
Phillips has made it clear to the studio that Phoenix is his top choice for the character. WB has yet to comment and negotiations are not yet underway.
The Joker origin movie is the first in a series that DC Films wants to pursue with a fresh take by focusing them in as solo projects instead of an interconnected universe according to Business Insider.
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To recall, it was a year ago when Warner Bros announced that it would be producing a solo "Joker" movie that will be separate from the studio's current brand of DC movies, the DCEU, which interconnects the story of other DC movies.
The film is a standalone project which will exist outside of the DC Extended Universe, which includes films such as Wonder Woman and Justice League, and will therefore not affect Jared Leto's incarnation of The Joker in the Suicide Squad movies. Phillips had allegedly met with Phoenix sometime in December to successfully convince the Inherent Vice actor to assume the role of The Joker.
And while Phoenix has turned down several other comic book cinematic roles including Lex Luthor in "Batman v Superman: Dawn of Justice", according to Variety, it would seem that he is keen to experiment with the Joker's character.
The Joker movie has been described as a gritty crime drama '80s film that will delve into what it took for the Joker to become a mastermind criminal. He was asked by Marvel to play the Hulk in The Avengers and was approached again for the titular role of Doctor Strange. He was most recently seen in the Gus Van Sant film "He Won't Get Far on Foot".