Maron is being considered to play an agent of De Niro's talk show in the film, helping to book Phoenix onto the program before he has become The Joker, Variety reported.
Joker, described as being a gritty character study with a dark tone, is set to arrive in theaters on Oct. 4, 2019.
The film will take place outside of DC's extended film universe and would help launch a new DC Comics banner at Warner Bros. that will allow the studio to tell unique comic book stories using different actors in the the title roles.
The Joker was last portrayed by Jared Leto in Suicide Squad who is expected to reprise the role in a standalone film that Leto is executive producing.
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