At this point in history, actor Heath Ledger is probably most remembered for his role as The Joker in the Christopher Nolan film The Dark Knight. Ledger's remembrance is closely tied to the movie because of the substantial rumors surrounding his mental state during, and immediately following, the filming process. The rumors question whether Ledger's commitment to The Joker drove him crazy or lead him to the drug addiction that eventually took his life.

Following Heath Ledger's death his father, Kim Ledger, worked with a German Documentary (EW) crew to reveal "The Joker Diary".  From the film: β€œHe pretty well locked himself up in a hotel, in his apartment, for a month or so, to sort of galvanize the upcoming character in his own mind, that was typical of Heath on any movie. He would certainly immerse himself in the upcoming character. I think this was just a whole new level.”

Kim Ledger gives the above quote while he is thumbing through the diary his son kept in preparation for his role as The Joker. The pages of the book show photos of Alex DeLarge from A Clockwork Orange, Joker playing cards, and panels from Joker comics that Heath was drawing on for inspiration.

From a 2007 interview with Empire, Heath Ledger describes his version of The Joker: "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 and 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 toward his acts. He's just an absolute sociopath, a cold-blooded, mass-murdering clown, and Chris has given me free reign. Which is fun, because there are no real boundaries to what The Joker would say or do. Nothing intimidates him, and everything is a big joke."

Ledger's performance as The Joker was incredible, controversial, and totally unique. Heath had taken the character in a darker direction than had been seen in years- especially in movies. The crowds loved him, comic fan boys questioned him, and he was awarded a posthumous Oscar for his performance.