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 A Clockwork Orange.
Even though Christopher Nolan offered her the part, Katie Holmes decided not to reprise her role as Rachel Dawes. Instead, she opted to co-star with Diane Keaton and Queen Latifah in Mad Money that same year. Sarah Michelle Gellar, Isla Fisher, Emily Blunt and Rachel McAdams were all considered for the role before Maggie Gyllenhaal stepped in.
Whilst filming in Chicago, Wanted was the neighboring production, and Morgan Freeman worked concurrently on both films. At one point, Wanted comic book writer Mark Millar visited the set but without permission. The security and Lauren Shuler Donner (who also visited the set at that time) caught Millar sitting on the Batpod. Millar was escorted away from the set.
This is the fourth time a film based on a comic has been nominated for a Best Supporting Actor Academy Award. The others are Al Pacino for Dick Tracy, Paul Newman for Road to Perdition, and William Hurt for A History of Violence.
Aaron Eckhart is technically the third actor to play Harvey Dent/Two-Face in a feature film. Billy Dee Williams played a pre-disfigured Dent in Batman, but he didn't become Two-Face until Batman Forever, where Tommy Lee Jones took the role (in that film, Dent's transformation was briefly shown in a flashback where Batman tried to save him from Boss Maroni, and Harvey was already Two-Face at the beginning). This is the first Batman film to depict Two-Face's origin, starting with Harvey Dent as Gotham's district attorney and eventually becoming Two-Face.
The character of Reese is an allusion to The Riddler, who attempts to reveal the identity of Batman. Much like Edward Nygma whose name sounds like "enigma" (as in E. Nygma), Mr. Reese sounds like "mysteries".