1. In preparation for his role as The Joker, Heath Ledger hid away in a motel room for about six weeks. During this extended stay of seclusion, Ledger delved deep into the psychology of the character. He devoted himself to developing The Joker's every tic, namely the voice and that sadistic-sounding laugh (for the voice, Ledger's goal was to create a tone that didn't echo the work Jack Nicholson did in his 1989 performance as the Joker). Ledger's interpretation of The Joker's appearance was primarily based on the chaotic, disheveled look of punk rocker Sid Vicious combined with the psychotic mannerisms of Malcolm McDowell's character, Alex De Large, from Clockwork Orange (1971).
2. During the scene where the Joker crashes Bruce Wayne's party for Harvey, when he first appears in the elevator Alfred was meant to have some lines, however this was the first time Michael Caine had seen Heath Ledger with the Joker make up on, you can even see the shocked expression on his face as the Joker walks past him.
3. Heath Ledger designed the Joker make-up himself, using white clown makeup and cosmetics from a drugstore, reasoning that since the Joker himself would design and apply it, Ledger should do so as well. Once his design was approved, the makeup team was responsible for replicating the look each day for filming.
4. In an interview, Heath Ledger said that the Joker was his favorite role.
5. Heath Ledger directed both homemade videos that the Joker sends to GCN himself. The first video involving the fake Batman was done under Nolan's supervision. Nolan thought Ledger had done so well with that sequence, he felt there was no need for him to be there when it came time to film the scene where reporter Mike Engel reads the Joker's statement. He put his trust in Ledger and let him do whatever he wanted, ultimately pleased with the result after he'd seen the outcome.
6. Bruce Wayne drives a Lamborghini Murcielago in the movie. The Spanish word for "bat" is "murciélago".
7. During the Hong Kong action scene (to bring Lau back), Batman shoots time-bombs on the glass which has a time of 2:22 minutes. The bombs explode almost exactly after 2 minutes 20 seconds in real time, which shows that the action could happen in almost 2 minutes.
8. Made more money than Batman Begins (2005)'s entire domestic run in only 6 days of release.