In a 2007 interview with Empire, Heath Ledger described how he prepared to play the iconic character and mentioned a diary that he used in the process:
It's a combination of reading all the comic books I could that were relevant to the script and then just closing my eyes and meditating on it. I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices — it was important to try to find a somewhat iconic voice and laugh. I ended up landing more in the realm of a psychopath — someone with very little to no conscience towards his acts. He's just an absolute sociopath, a cold-blooded, mass-murdering clown.
Heath Ledger's father, Kim Ledger, takes the viewer through the Joker diary.
"He pretty well locked himself in a hotel room for weeks," Ledger recalled of his son's preparation for the film.
"He galvanized the upcoming character. That was typical of Heath. He would do that. He liked to dive into his characters, but this time he really took it up a notch."
Jesters, clowns and jokers appear among the images in the diary, as do pictures of actor Malcolm McDowell as Alex in A Clockwork Orange.
And, of course, images of THE Joker from the Batman comics.
“The hospital scene is interesting because when he was a kid, his sister Kate liked to dress him up as a nurse," Ledger said. "He looked pretty funny like that. He looked pretty funny in the film, too.”
The actor wrote out his lines for the hospital scene by hand in the diary.
The Joker: You know. I don't want there to be any hard feelings between us, Harvey. When you and, uh...
Harvey Dent: Rachel!
The Joker: Rachel were being abducted. I was sitting in Gordon's cage. Now, I didn't rig those charges.
Harvey Dent: Your men. Your plan.
The Joker: Do I really look like a guy with a plan? You know what I am? I'm a dog chasing cars. I wouldn't know what to do with one if I caught it! You know, I just... do things.