a while ago i read on wiki that what ledger did was lock himself away in some hotel for two weeks where he read The Killing Joke and Arkham Asylum, kept a diary as the joker, came up with the walk and voice, and was influenced by Alex from Clockwork Orange and Sid Vicious of the Sex Pistols in developing the persona. he also forced himself to stay awake for long periods of time to help him feel more ka-razy like the joker would feel.