I've sometimes heard it said that when you're watching documentaries and you're seeing footage of B-52 flying in formation or soldiers going over the top, what you're very often seeing is actually just stock footage from training exercises or staged events put together for newsreels and the like. Not all of it, obviously, but that does make a lot of sense, if only as a matter of convenience.
So how much do documentaries tend to rely on footage like that versus actual footage from the field?
While only tangential to your question, I think this will likely still be of interest as it does discuss the nature of combat footage, although doesn't get deep into staging of it.