Visible crew/equipment: During Batman's interrogation of the Joker, when he picks him up and slams him against the wall, for a very brief moment you can see the camera and the cameraman in the reflection of the mirror on the right. (01:29:10)
Visible crew/equipment: In opening scene of the aerial view of the building adjacent to the bank, you can see the reflection of the filming helicopter on the left side of the building. (00:01:10)
Visible crew/equipment: In the scene where the convoy is driving through the entrance of the tunnel you can see the black helicopter the film crew uses right behind the police helicopter. (01:15:15)
Visible crew/equipment: When the Joker gets on the bus, after blowing up the hospital, about halfway along inside the bus is a large camera set-up. It can't belong to Mike Engel's news crew as it is not only too large to be hand held or shoulder mounted (it is one of the IMAX cameras used to film a shot from within the bus that wasn't used in the final cut) but also has a thick black cloth draped over it for the cameraman to work beneath. Obviously not a camera used by any news crew. (01:48:00)
Visible crew/equipment: When the Joker places the smoke grenade in the bank manager's mouth, a wire or tube can be seen on the left side of his face. It's only visible in the shot when the string pulls the pin, as soon as he drives away in the bus.
Answer: It's habit, and a sensible one at that. When he's in the mask, when he's being Batman, he uses the voice, even if the person he's with knows who he is. If he drops back to his normal voice with some people, it sets the precedent for using it while wearing the cowl, which means that he's more likely to slip up and use it around people who don't know, potentially revealing his true identity. If he sticks rigidly to using the voice when kitted up as Batman, regardless of situation, it minimises that possibility.
Tailkinker ★