Jack Nicholson: I had a metaphor in my mind for the character. The shorthand name that I came up with was Velvet Death. (Newsweek, June 1989)
Metaphysically, The Joker was dipped in chemicals and lost his mind - not unlike the rest of society. He has his identity melted into his brain. He flows with the corrosion, so to speak.(Time, June 1989)
In The Dark Knight, Joker experiences a different kind of insanity than the comic book Joker. He isn't a prankster. He doesn't joke or laugh all the time. He doesn't have the sick Daffy Duck side to him. He is completely insane and out of his mind but in a different way - for him everything is a game that has no real purpose other than careless fun. He has absolutely no remorse or limits, and he does get fury attacks at times.
The Joker observes no rules, pursues no grand scheme; he's the terrorist as improvised artist. Why, he might blow up a hospital or turn ordinary people into mass murderers to save their own lives. - Time review, June 2009