The interrogation scene really splits The Dark Knight in two; we've reached the point where another less daring superhero movie would end (with its Harlem brawl The Incredible Hulk did) and are now poised to leap into the psychotic explosion of madness the Joker has set up. Its a cracking sequence that really pushes just how far the Joker can go. And, it seems from behind the scenes stories, Heath Ledger too.
In a shocking pre-filming aside that rivals George Lucas telling Mark Hamill just who Darth Vader was, Heath Ledger asked Christian Bale to hit him as hard as Batman would. A pretty startling ask made only more amazing by the criticism of many fans against The Dark Knight Rises that it was clear the blows weren't hitting.
This is the purest example of the lengths Ledger was willing to go for authenticity in the character, showing a devotion that most method actors would baulk at, and it more than shows up on screen.
Know of any more Heath Ledger-isms that he incorporated into his Joker? Tell us them in the comments below.