It was a rehearsal too. Why did Baldwin pull the trigger and shoot her in the chest while only rehearsing the scene?
Because he is a killer.
If it was a rehearsal, they probably told Baldwin to do it again and it set off the jackass's anger and he pointed the gun to dry fire it at them for intimidation. Whoops! I'll bet they weren't behind the camera, but off to the side at a monitor. That's a double whoops.