15Ledger Locked Himself in a Room for Over a Month
Heath Ledger wanted to ensure that his version of the Joker was a fresh and distinct variation that movie-going audiences had never seen before. Both Jack Nicholson and Cesar Romero had played the classic Batman villain in film adaptations, but Ledger wanted to explore a side of the Joker which
was present in the comic books and graphic novels but had never made its way to film. Ledger came onto Nolan's project in the very early stages, and so he was able to take his time before the production to explore the Joker.
In order to get into the mind of the Joker, Heath Ledger spent around six weeks in an isolated hotel room. During this time, he wrote in a journal, looked at source material, and practiced different voices. Ledger wanted to make sure that the Joker's voice and laugh was perfect before he began filming. The resulting Joker was a sociopathic clown without any empathy or remorse.