What could be the major factor that convinced the casting directors of The Dark Knight to decide that Heath Ledger will play The Joker's role?
Despite endless speculation on which actor had been chosen to portray The Joker, Heath Ledger had always been Christopher Nolan's one and only choice for the role. When asked the reason for this unexpected casting, Nolan simply replied, "Because he's fearless." He later said that the two had met up to discuss it before there was even a script for the film.
Here is a good interview I found which reveals how was Heath Ledger selected by Nolan. He gave this interview for an hour-long Q&A at the Film Society of Lincoln Center in New York City. There he discussed about the influence James Bond films have had on him, his experience working with Heath Ledger, his passion for pioneering IMAX technology and producing next year's Superman reboot, "Man of Steel."
In that interview he shared how he chose Heathe Ledger:
On casting the late Heath Ledger as the Joker:
Really, in a sense, Heath chose me. I met Heath for a couple of films -- I actually met him in relation to Batman, as well, because I was meeting kind of every young actor. He very graciously came and met me for a drink and began to explain why he would never do this kind of movie. [Laughs] But he was very polite. He was really just a lovely guy, and so I thought, "Well, shame I can't convince you, but this is what we're trying to do with this thing.' I think when he saw "Batman Begins," he'd probably remembered the things I'd said about what I was going to try to do, and he felt that I'd done it, so I met with him for the Joker. I didn't know whether it was something he'd be interested in, but I sat and chatted with him in my office for a couple of hours. We didn't have a script at that point, but -- my brother was writing it at the time, and we knew kind of where that was going to go, and it was very much what Heath had in mind. And he just was determined to do it. He just had a vision for something, and the way he termed it to me at the time was, he really didn't like to work too much. He liked to do a character and then stop working then let enough time go by. He wanted to be hungry for it. And when he came to me, he was clearly in that state: Very hungry. He was ready to do something like that and just own it -- which is what he did.
So this is how the casting of Heathe Ledger was done as the Joker. Also he shared on the process preparing for and the reality of Ledger's transformation into the Joker and on the technical (and communicative) difficulties behind Ledger's first scene as Joker in the same interview.
From this interview of Nolan at IGN, I am sure that Nolan himself wanted to cast Heath Ledger as Joker. He says that he wanted to work with Heath on past projects, but could not. He and Heath shared a character portrayal of Joker and that's how he was cast in the movie.