A part of the reason that Joaquin Phoenix was cast for the role of the Joker was because he helped influence the idea, which bought about the movie. About three to four years prior to the movie being released, Joaquin Phoenix called up his agent.
During the call, he introduced the idea of taking one of the DC characters and make a low budget film about them. He suggested the film be a character study and that it should be one of the villains. Knowing about this call, the director of the film began writing the script with Phoenix in mind.
Three or four years ago, Phoenix called his agent and suggested that they take one of these characters and produce a lower budget film about them. He said it should be a movie, but more of a character study, and he suggested using one of the villains.
So, Todd Phillips, the director had Joaquin Phoenix in mind since he began writing the story. He also knew that Phoenix was capable of playing such a dark and multi-layered character. Phoenix has also played some very unique and exciting roles.