3. That Tongue Stick Out Was Ledgers Own Personal Tic
Sit down and watch some real life footage of Heath Ledger and the acting achievement of the Joker is only heightened; they have next to nothing in common. The accent, the demeanour (Ledger always seemed pretty chilled in interviews) and obviously the look (its not just the prosthetics; his entire face changes when in character) are so distinct that its hard to think of them as the same person, an effect helped by Ledger not wanting any behind the scenes footage of him being shot.
Theres only one thing really liking them that's so subtle you probably havent even noticed it before; they both have a tendency to stick their tongue out while speaking. With the Joker it may be a lot more pronounced - check it out in the almost nervous way he prepares himself to torment Rachel - but its born out of the actors own tic.
Given that with Ledger its little beyond a lip smack, its hard to notice, but its most certainly an adaptation of his own habits to make the Joker feel like a real person.