This entire conversation begs the answer to one potential scenario:
You're in line at a conveinence store. A cop shows up in a plain car, and runs in for a can of skoal. You exchange niceties. Hello, how ya doing. Etc...
While ya'll are standing there in line, three arm thugs bust in, unaware of the cop standing in line behind you. They hollar "this is a stick up!"
If the cop starts shooting, can you as well? To offer help, especially if he's outgunned? Should you?
I think if this happened I'd just yell "cut!" because obviously you're on the set of a bad movie or TV show.
Bryan