Ledger, who won a posthumous Oscar for his turn as The Dark Knight villain Joker, initially told Nolan that he wasn't interested in appearing in a comic book blockbuster. Christian Bale was eventually cast as Batman in 2003.
"He was quite gracious about it, but he said, 'I would never take a part in a superhero film'," The Playlist quotes Nolan as saying. "I explained to him what I wanted to do with Batman Begins and I think maybe he felt I achieved it."
Nolan added that he helped Ledger prepare for the role of the anarchic Joker by sending him books containing Francis Bacon paintings and Anthony Burgess's novel A Clockwork Orange.
"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," the director said.
Nolan also spoke about the unpredictability of Ledger's performance, and how he developed the character's unique voice from camera tests through to the actual production.
"The voice was certainly scary because it would shift in pitch," he recalled. "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."
Watch Christopher Nolan's The Dark Knight Rises interview with Digital Spy below: