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.