I would say that Ledger doesn't even look like Ledger except for the interrogation scene. His distinctive cheek bones and jaw are really camoflagued by the scars and the smile.
I still think though, that if Ledger is nominated for an Oscar, it's best SUPPORTING actor. There isn't a LEAD role here. Javier Bardem got as much screen time in NO COUNTRY FOR OLD MEN and it's 'supporting'. This is an ensemble cast.
Secondly, if someone from TDK is nominated, it should be Gary Oldman. Ledger is SUPERB. Eckhart is WONDERFUL. Gyllenhaal is TERRIFIC. Bale (with the exception of the growl) is AWESOME.
But Gary Oldman *IS* Gordon in such a way that you don't even NOTICE him. He's a totally convincing performance. He's a huge part of the film, a major character who, when killed, is a HUGE turning point yet - he gets no cool lines. He does nothing cool. He doesn't have a cool costume, no cool makeup, and has not got a cool voice. He's just Jim ****ing Gordon.
Ledger is very subsumed in the role in a terrific fashion. Of the three costumed guys, he's the most powerful, mainly in part because the character is written so well. BUT! Ledger, while not appearing as "Ledger" appears as an actor with one hell of a performance. Your kinda aware he's an actor.
But Oldman is Gordon. Scarily so. So much so I have to remind myself he's Gary Oldman, he's so completely invisible.
To put it another way: Oldman has far, FAR less to work with than any of the big three, yet he does almost more than any of them. Think about what HE has compared to what the others have, and then realise just how indispensable his performance is and you'll see why I think his performance is the best.
But! This is like calling a photo-finish in a horse race. We're talking nanoseconds because all of these guys are terrifically talented. I think Oldman just outdoes the rest, but only just. Any of these guys performance in another film would be, without a doubt, the highest peak of the film.
They're all immensely impressive.