People comparing the odd casting of Cillian Murphy and Liam Neeson are missing two key aspects of that decision: both are well reputed as good actors, especially Neeson (Also note, when the movie was solicited, Neeson was listed as Ducard, not Ra's), who had the significantly bigger role. Heath Ledger is...not so good. Anyone seen the Order? The Patriot? A Knight's Tale?! I have faith in Chris Nolan, but I have absolutely none in Heath Ledger.
In response to Johnny, remember, you're dealing with Wikipedia here, which tends to be unreliable on occasion. You actually would've saved yourself a question if you'd checked the listed source:
http://forum.newsarama.com/showthread.php?t=90305&highlight=Heath+Ledger"HL: The Killing Joke was the one that was handed to me. I think it’s going to be the beginning of The Joker. I guess that book explains a little bit of where he’s from but not too much. From what I’ve gathered, there isn’t a lot of information about The Joker and it’s left that way."
As of that interview, he hadn't even read it yet. Considering the comic in question is about a half hour to forty-five minute read at best, that screams a lack of dedication to me.
Also from that same interview is the answer to your question: "DRE: Have you read many Batman comics?
HL: No and I think that’s kind of helping me a little bit. I was never really a fan of comic books or comic book movies. I never despised them but I was never one to read them. I never sought out the films but I would sit down and enjoy them. So because of that I really feel that I’m not carrying much pressure."
I've grown up reading this stuff and he seems to be carrying in almost the exact same attitude Halle Berry carried into X-Men, and she was, by far, the worst person in that cast, almost definitely because she didn't care about the source material.