It is sincerely my humble opinion that one of the actor’s duties is to enlighten us. By this I mean a process of driving the viewer through his psychological journey in a convincing, fascinating way with the aim of fulfilling our basic need for transgression. We want to see our ideal selves, to identify and be inspired.
In Heath Ledger’s performance for The Dark Knight, a cherished, awarded and widely accepted performance to say the least, I’m afraid I’d have to argue against. For he (the actor) presents an utterly problematic image, the reasons being purely bad acting choices and, I would add, a few psychological complexities. 
He let the character take over the actor, which would certainly be a good thing in another case, but in this case becomes too much. A film performance which successfully presents the dark side of the soul should at least provide us with little signs of positivity, little signs of an actor’s existence behind the curtain, little hints of genius. They help the actor guide us through his perception of the role’s complexities. Whether Heath Ledger did it on purpose or was just ‘carried away’ I don’t know, but I believe the result will only appeal to those who watch the film out of curiosity.
When asked for suggestions on how to judge acting, George C. Scott said “I have three tests: First, which dominates, the character or the actor? With very few exceptions it should be the character. Second, on film — as opposed to stage — we’re pretty much playing basic emotions: love, anger, fear, pity. So the trick is whether you can come up with any fresh choices to present these common emotions. Third — and this is the quality that separates the great ones from the good ones — I look for a ‘joy of performing’ quality.” I see no joy in Ledger’s performance.
Read also
The Dark Knight at IMDb
The Dark Knight at Wikipedia
Heath Ledger at Wikipedia
The Dark Knight at Rotten Tomatoes
The Dark Knight (awards won and nominated for) at IMDb
Susy
how you don’t see a joy in Ledger’s performance?…this article doesn’t make any sense..also the point of a great performance is when the actor totally dissapears into the character.. Ledger did just that
Angeliki Coconi
I agree with you Susy. I thought Ledger’s performance was incredible, original and so different from anything we’d seen.
NickPappas
I believe a disappearance into a character is allowed within the limits of mental health and balance. That’s all. He didn’t really look so healthy in my eyes, which is also proven by the fact that he later commited suicide. Art should enlighten us, even evil characters should contain hints of mental balance.