Phoenix, 43, has previously said he wasn't against taking on the role of the Joker after reports named him as the next actor to personify the sinister comic character.
Do you think Phoenix will make a great Joker? Phoenix entered talks to take on the role this past February, but this was far from a done deal as the Oscar-nominated actor had previously been in prolonged talks to lead Marvel's Doctor Strange before ultimately backing out.
Studio bosses describe the plotline as an "exploration of a man disregarded by society (that) is not only a gritty character study, but also a broader cautionary tale".More news: Samuel Umtiti heads France into World Cup final as Belgium fall short
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The director attached to the project, Todd Phillips, may come as a surprise, having most famously helmed the Hangover Trilogy and co-written the Borat movie along with Sacha Baron Cohen.
Shooting on the new film is expected to begin as early as September.
Tethered with a budget of $55 million and more expectations than we can count, the Joaquin Phoenix-fronted Joker movie is officially headed for a production start this fall.