A German documentary about Heath Ledger has revealed the diary that the actor used prepared to play The Joker in The Dark Knight.
In a 2007 interview, Heath Ledger 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