Joaquin Phoenix will play the Joker. Imagine for a moment Phoenix's Commodus from Gladiator speaking with the Joker's singsong voice, and then try not to smile. Phoenix joins a long line of brilliant actors who have played the Clown Prince of Crime.
And as is appropriate for a movie that has Martin Scorsese as one of its producers, Rober De Niro will be in the movie as "a talk show host who is somehow instrumental in the Joker’s origin," according to THR.
Marc Maron (who has been nothing but brilliant on GLOW) is in the movie as "an agent on Robert De Niro’s talk show who plays a part in booking Phoenix’s character, and eventually causing him to go mad and become the Clown Prince of Crime." (via Variety)
Zazie Beetz will play "a single mother who catches the interest of the man who will become the clown prince of crime." (via THR)
The Wrap reported that Frances Conroy will play the Joker's mother, Penny. They also report that Zazie Beetz will play "a single mother who catches the attention of the Joker prior to his transformation into the nihilistic Gotham City villain," which might seem to offer some echoes of The Killing Joke by Alan Moore and Brian Bolland.
There had been plans for Alec Baldwin to play Batman's father, Thomas Wayne, but for whatever reason, they fell through. It might be for the best.
(Den of Geek US)
It was previously revealed that Phoenix’s character would be named Arthur Fleck in the film.