Taking on the role of the Joker from the 'Batman' universe isn't an easy proposition after Heath Ledger gave such a definitive performance in 'The Dark Knight'; just ask Jared Leto, who received rough reviews for his own interpretation in 'The Suicide Squad'. Joaquin Phoenix is the man who's been brave enough to try. He's a high-quality actor with an impressive track record, so if anyone can do it, we're backing him.
This is an origin story for Gotham's most famous villain, focusing on how Arthur Fleck transitioned from failing nightclub comedian to psychotic mega-villain, by way of a stay in Arkham Asylum. There will be no Batman appearance in the film at all, other than an apparent cameo from a young Bruce Wayne, so the stage is set for the Joker to have his evil fun without interruption. Given the fan base that the Batman universe has, this is a surefire hit, and should be an intriguing watch.