What did Heath Ledger do to prepare for his legendary Joker performance?

The answer

For The Dark Knight (2008), Heath Ledger isolated himself in a London hotel room for approximately one month, immersing himself completely in the Joker's psychology. He kept a detailed character diary filled with comic book clippings, playing cards, handwritten copies of the Joker's dialogue, and disturbing images that revealed his deep psychological excavation of the character. Director Christopher Nolan noted that Ledger studied A Clockwork Orange by Anthony Burgess and paintings by Francis Bacon. During isolation, he experimented with dozens of voice variations to create the Joker's unsettling vocal quality. Ledger's father Kim later described the diary as being 'on a whole new level' compared to his son's usual preparation methods. Ledger won a posthumous Oscar for the role.