4. The Clapping Was Famously Improvised (But Made Iconic By The Trailer)
OK, this isn't really one you could have escaped before given how often it does to rounds, but at the same time it really should be included. That iconic, sarcastic clapping at Jim Gordons promotion stuck with fans because it was so emblematic of the character and it brilliantly came entirely from Ledger on the spot. Its odd to think of the scene ending rather ignominiously without it, but really stands as testament to how much Ledger understood the character.
One funny little tidbit about this moment you may not have noticed before is how it appears in the trailer; coming at the climax, the Jokers clap that signals in the crescendo of Hans Zimmers theme. Check it out below (its at 1:55 in the video). It may not say much about the character but it explains why the pre-release material felt so fresh (and how it was iconic even before the film hit cinemas).