“He was quite gracious about it, but he said, 'I would never take a part in a superhero film,'" Nolan told last night's audience according to indieWire. He explains that the time between Batman Begins and The Dark Knight apparently changed Ledger's mind, becuase, “I explained to him what I wanted to do with Batman Begins and I think maybe he felt I achieved it.”
Once The Dark Knight was going forward, Nolan contacted Ledger again, even though he'd been rebuffed the first time, and following Batman Begins, Ledger was ready to go forward even before a script was written.
"Heath spent months and months [preparing],we cast him even before the script was written so he had a very long time to obsess about it, think about what he was going to do, to really figure it out,” Nolan said, adding that he sent Ledger quite a lot of material to help him prepare, including a copy of A Clockwork Orange.
The director also confessed that neither he nor Ledger were completely sure at first that what they were doing with The Joker would work as well as they hoped.
“The voice was certainly scary because it would shift in pitch," Nolan said. “You never quite know which way the pitch is going to go with the voice. Just as the physical movements were [unpredictable], you didn't know what he was going to do with his hands, the way he moved, it was always a surprise. The actual tone of his voice was a surprise too. Sometimes threatening and sometimes more sing-song and light."