If you thought people accidentally socking each other in genre shows and movies was a distinctly 2017 phenomenon, well, turns out stars have been actually smacking each other around on screen instead of pretending for a while. But in The Dark Knight's case, it wasn't accidental: it was at the request of Heath Ledger.
The tidbit, resurfaced in the new book 100 Things Batman Fans Should Know & Do Before They Die by Joseph McCabe, revolves specifically around Batman and the Joker's tense encounter in an interview room at Gotham City Police Department during the movie. According to Christian Bale, it was the first scene he actually shot with Heath Ledger on the film, and his co-star was already well into the swings of getting into Joker's character.
So into the swings that, when it came for Batman to throw the Joker about the room, according to Bale, Ledger wanted him to really attack him:
Our first scene was in an interrogation room together, and I saw that he's a helluva actor who's completely committed to it and totally gets the tone that Chris [Nolan] is trying to create with this. We're not going for actors revealing their enjoyment of playing a wacky caricature. We're treating this as serious drama. You go into character and you stay in the character. I love that. I find that so ridiculous that I love it, and I take that very seriously. Heath was definitely embracing that. When he was in the makeup and the garb he was in character the whole time; and when he took it off he was absolutely fantastic company to be around.
As you see in the movie, Batman starts beating the Joker and realises that this is not your ordinary foe. Because the more I beat him the more he enjoys it. The more I'm giving him satisfaction. Heath was behaving in a very similar fashion. He was kinda egging me on. I was saying, "You know what, I really don't need to actually hit you. It's going to look just as good if I don't." And he's going, "Go on. Go on. Go on…." He was slamming himself around, and there were tiled walls inside of that set which were cracked and dented from him hurling himself into them. His commitment was total.
So it wasn't just the Joker egging Batman on in that scene, it was Ledger egging on Bale. The actor doesn't say if he really ended up going all the way, but the rest of the scene, when the Joker gets hurled about the room and into walls, windows, and tables, that was all Ledger's enthusiastic performance. That's one hell of a commitment.