OK, I decided to "acquire" a camcorder boot of Dark Knight to get to the bottom of this. Asfar as I can tell, this is what people are saying is what originally happened in theatres:
"In the cinema version heath ledger puts the Stanley knife in the hoodlum's mouth and from a reverse shot the viewer sees the joker thrust his hand. The reverse shot was quite effective because you only saw heath moving his arm then the reaction of the other hoodlum as the body fell to the floor. "
This clearly doesn't happen in the boot I checked (which was a cinema rip as you can hear the audience at the start). What happens is Joker delivers his speech with the knife in his victim's mouth, then he says "Why so serious" and we cut instantly to the guy's friend who is being restrained, we get an audio cue from the score that suggests the Joker has slashed his victim's mouth while we watch his friend flinching heavily (actually thrusting forward), then it cuts to the Joker letting go of his victim, and his limp body falling to the floor.
There is no thrusting motion, it's actually the flinching of the victim's friend and the score that suggestion any motion/slashing at all. This is a common and effective trick filmmakers use when they want to imply a violent act without actually showing it.
