4- To prepare for the role of joke Heath Ledger locked himself in a motel room for 6 weeks and developed the sadistic laughing sound and the joker’s voice.
5- During the hospital scene, when The Joker is dressed as a nurse, his name tag reads Matilda, after Heath Ledger’s daughter Matilda Ledger.
6- As the Joker Heath Ledger scared Michael Caine (Alfred) so bad that he forgot his lines.
