Qual é o principal motivo para lançar Heath Ledger como The Joker?

De IMDB:

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.

Aqui está uma boa entrevista que encontrei, que revela como Heath Ledger foi selecionado por Nolan. Ele deu esta entrevista para uma hora de duração Q & A na Film Society of Lincoln Center, em Nova York. Lá ele discutiu sobre a influência que os filmes de James Bond tiveram sobre ele, sua experiência trabalhando com Heath Ledger, sua paixão pelo pioneirismo na tecnologia IMAX e produzindo o Superman do ano que vem, "Man of Steel".

Na entrevista ele compartilhou como ele escolheu 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.

Então é assim que o elenco de Heathe Ledger foi feito como o Coringa. Também compartilhou sobre o processo de preparação e a realidade da transformação de Ledger no Coringa e nas dificuldades técnicas (e comunicativas) por trás da primeira cena de Ledger como Joker na mesma entrevista .

Desta entrevista de Nolan na IGN, eu tenho certeza que o próprio Nolan queria lançar Heath Ledger como Joker. Ele diz que queria trabalhar com Heath em projetos anteriores, mas não conseguiu. Ele e Heath compartilhavam o retrato de um personagem de Joker e foi assim que ele foi escalado para o filme.