Actor/musician Dwight Yoakam turned down two different roles in this film due to scheduling conflict. One was for the part of Det. Wuertz (later played by Ron Dean), as well as the small part of the Gotham National Bank manager (portrayed by William Fichtner) featured at the very beginning.
Mark Ruffalo auditioned for the role of Harvey Dent.
Matt Damon turned down the role of Harvey Dent due to scheduling conflicts.
Many actor expressed interest in the role of the Joker including Paul Bettany, Lachy Hulme, Adrien Brody, Steve Carell, and Robin Williams (who also lobbied hard for the role of Joker in the 1989 Burton version of Batman which eventually went to Jack Nicholson).
Ledger reportedly called actor Jack Nicholson, who previously played he Joker in Burton's version of Batman, in order to reassure the actor that his version of the Joker will remain untouched and in no way would he [ledger] try to recreate or imitate Jack's performance. upon hearing about the death of Heath ledger Nicholson was quoted saying "I warned him".
In addition to reading The Killing Joke Ledger was also given Arkham Asylum: A Serious House On A Serious Earth which "really tried to read but had put it down" probably due to it's very graphical content.
The idea for Joker to change his origins story every time he told it was most likely taken from the Batman graphic novel Batman: The Killing Joke in which Joker is quoted saying "I'm not exactly sure what happened. Sometimes I remember it one way, sometimes another... If I'm going to have a past, I prefer it to be multiple choice!". Heath Ledger read that graphic novel as a reference when developing his version of Joker.
Heath Ledger's clown mask during the bank robery was designed after a mask worn by Ceaser Romero dressed as Pagliacci in an episode of the classic 1966 episode titled "the Joker is Wild."
Besides Christian Bale having to growl his lines during scenes, it was further digitally modified in post-production to make it sound more menacing.
The older gentleman who confronts the Joker in the fight at Bruce Wayne's penthouse is US Senator Patrick Leahy, D-Vermont.
The date seen on Gordon's security camera photo of The Joker taken during the bank heist reads "2008/07/18", which was the U.S. theatrical release date of the film.
Heath Ledgers habit of licking his lips throughout the movie as The Joker originally began due to the prosphetic make up irritating his mouth.
Christopher Nolan noted that one of his most distinct memories of the shooting of The Dark Knight was of Ledger visiting the set on a skateboard on his day off shooting, just to take in the magic of making movies.
Heath Ledger passed away prior to the release of The Dark Knight, making his Academy Award a posthumous win. Christopher Nolan has gone on record to specify that Ledger has completed filming prior to his death and that his role in the film was unaltered.
'The Dark Knight' is the highest domestic and worldwide grossing film of 2008.
Heath Ledger began compiling a "Joker diary" four months before filming began. It is composed of images and thoughts important to the character, including a list of things that the Joker would find funny. AIDS was one of the items on the list.
Some filming and pyrotechnics work was done at London's famous Battersea Power Station and prompted police calls from locals who thought terrorist bombs had been detonated in the city.
Though Heath Ledger's makeup looks extensive, the entire look took less than an hour to apply thanks to new prosthetics techniques.
Gotham City license plates are inspired by the Illinois license plate design.
A number of off-duty Chicago police officers were cast as Gotham City police extras.
In preparation for his role as The Joker, Heath Ledger sequestered himself away in a hotel room for six weeks while he developed the mannerisms for that character. He claims to have based it on a combination of punk rocker Sid Vicious and Malcolm McDowell's character from A Clockwork Orange.
Aaron Eckhart got the role of Harvey Dent based on his performance in the movie Thank You for Smoking.