Heath Ledger met with Christopher Nolan about the part of Batman and Bruce Wayne.

Before you get too excited, Nolan said, as is routine, he met with many young actors for the part. And while Ledger took the meeting, he immediately said he wasn't interested. “He was quite gracious about it, but he said, 'I would never take a part in a super hero film.' " Nolan said. The filmmaker said he thinks Ledger changed his mind eventually because, “I explained to him what I wanted to do with 'Batman Begins' and I think maybe he felt I achieved it.” - indiewire

After that initial rejection from Ledger Nolan was not deterred and he sought out the young actor for the role of Joker in. "He didn't like to work too much," Nolan said. "He liked to do a character and then stop working and let enough time go by until he was hungry for it again. And that's what happened when he came in; he was really ready to do something like that."had convinced Ledger that Nolan's films were a cut above the usual comic book movies and signed on tobefore a script was finished. "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 fed into Ledger's obsession by sending him material that might be helpful for his research into The Joker. He gave him Anthony Burgess's novel "A Clockwork Orange" and had him look at artwork from Francis Bacon, the painter.Nolan acknowledge that Ledger did a lot of experimenting with pitch and tone of The Joker's voice. He even spent a lot of time trying out different hairstyles and picking from an assortment of rubber knives."Like a lot of artists, he would sneak up on something," Nolan said. "So you couldn't really sit and go, 'Okay, you're going to do the Joker. You're going to show me what it's going to be.' You had to sort of say, 'Let's read this scene. Don't act it, just read.' And he'd sit with Christian and there would be a line or two where his voice was a little different, throw in a little bit of a laugh. And then we would film hair and makeup tests and try different looks, and in that, he'd start to move, and we'd have these rubber knives and he'd choose what weapon and explore the movement of the character. We weren't recording sound, so he felt quite able to start talking and showing some of what he was going to do. And in that way he sort of sneaked up on the character."