To prepare for his role as the Joker, Heath Ledger lived alone in a hotel room for a month, formulating the character's psychology, posture and voice (the last one he found most difficult to do). He started a diary, in which he wrote the Joker's thoughts and feelings to guide himself during his performance. He was also given Alan Moore's comic "Batman: The Killing Joke" and "Arkham Asylum: A Serious House on Serious Earth" to read. Ledger also took inspiration from Alex in A Clockwork Orange(1971) and Sid Vicious.
When Gordon is promoted to Commissioner, Heath Ledger improvised the Joker joining in on the applause.
Michael Caine had never met Heath Ledger before. He later said of the Joker's arrival at Bruce Wayne's fundraiser: "I'd never seen any of it and the elevator door opened and they came out and I forgot every bloody line. They frightened the bloody life out of me."
Heath Ledger became very attached to his role. He would show up on the set, in full costume, even on the days he wasn't filming.
Heath Ledger was given an advanced screening of the opening bank heist scene. That six minute sequence was the only part of the completed film he saw before his death.
In the banquet scene, when the Joker and his troop arrive, the man that the Joker says reminds him of his father is Senator Patrick Leahy of Vermont. Senator Leahy is a huge Batman fan and has lent his voice to the animated series and also had another small cameo appearance in Batman and Robin.
When the Joker crashes Bruce's party and threatens Rachel with the knife, it was the first time Maggie Gyllenhaal had seen Heath Ledger in his make-up. When the Joker cups her face in his hand and says "look at me", that was improvised by Ledger because Gyllenhaal was so frightened by his appearance that she was trying to look away.
Heath Ledger found inspiration for his portrayal of the Joker from Sex Pistols bassist Sid Vicious, among other sources. Gary Oldman (who plays Commissioner Gordon in the film) played Vicious in the biopic Sid and Nancy.
This is the first Batman film that doesn't include the word "Batman" in the title.
The Dark Knight made as much money in six days as Batman Begins did in its entire domestic run.
Because of a ruling shortly before the Academy Awards ceremony, Heath Ledger's daughter Matilda will gain possession of his Oscar statuette on her 18th birthday in 2023.
This marks the second time movie characters adapted from another source have lept off the Two-ifc, the world's third or fourth tallest building, in Hong Kong. (The similar-looking building that Batman enters is the building's shorter and earlier-constructed twin, the One-ifc.) The first was in Tomb Raider: The Cradle of Life in which Lara and Terry Sheridan leap off the still-under-construction building to escape their pursuers.
In the restaurant scene, right when it begins with Harvey and Rachel talking, if you look to the bottom right of the screen, Heath Ledger, who is not wearing the Joker makeup, is sitting at a table with someone. [This is up for debate, as it's a strong likeness but not definitively him. Regardless, worth a look.]