De uma entrevista com Christopher Nolan:
I don't really know where that came from... He actually had a sort of prosthetic appliance for the scars that wrapped over his lower lips, and the first couple of scenes where he was playing it that way, I actually had thought it was a mistake... I thought the prosthetic was coming loose on the bottom lip and he was sticking it down with his tongue or something.
And it was only after we shot a couple of scenes I realized oh no this is a thing he's found, this is a sort of... I think it's a lizard like... or serpent like elemental kind of gesture.. and I think it works... I think it's very creepy...
[Click here to listen to Nolan talk about Ledger's creepy lip-smacking]