This will be a hard film to watch for many movie-goers because of today's death of Health Ledger.
This is posted on the trivia section of IMDB.com, but who knows what bearing it had:
The Joker's character is based on his first two appearances in the comic books, as well as his portrayal in the graphic novel "The Killing Joke." To prepare for the role, Heath Ledger lived alone in a hotel room for a month researching the character and developing his performance, which he claims is based upon Sid Vicious and the character of Alex in A Clockwork Orange (1971). Ledger found the role extremely difficult, and suffered insomnia as a result.
This was also posted as a quote by the director of the film: