Question, could you partially pull back the hammer on the revolver just before it locks and then release it and make the gun fire?
In that situation, normally the half-cock notch in the hammer would catch the hammer and prevent the firing pin from striking the primer (as long as the trigger is not already being pressed). However in practice half-cock notches are not 100% reliable in that situation, and in my experience they are more prone to wear than most other parts of the firing mechanism.
On a single-action revolver the half-cock position is where the hammer is locked partway back, and cannot be dropped by pulling the trigger. The half-cocked hammer position gets the hammer out of the way of the cylinder, and unlocks the cylinder, so that it can be easily rotated by hand. The half-cocked condition is used when you load and unload the cylinder, as the cylinder does not swing out on most single-action revolvers. In the half-cocked position the cylinder is easily rotated by hand and the cartridges can be inserted or removed one at a time to load or unload the firearm.
In order to return the half-cocked hammer to the normal hammer-down condition, the hammer is carefully thumbed back a bit beyond the half-cock position, then with the thumb STILL preventing the hammer from falling, the trigger is pressed, and the thumb EASES the hammer down. DURING THIS ENTIRE OPERATION THE FIREARM SHOULD BE POINTED IN THE SAFEST POSSIBLE DIRECTION, like maybe at the ground a few feet in front of the person handling it. Lowering the hammer on an empty chamber is standard practice to prevent an accidental discharge from your thumb slipping off the hammer when lowering it with a single-action revolver.
My understanding is that single-action revolvers have a considerably higher incidence of accidental discharge than either double-action or semi-automatic actions. The above is probably one reason for that. Another is idiots attempting a "quick draw" who have their finger on the trigger too early in the drawing cycle, such that the trigger is already pressed by the time they finish cocking the weapon, so it fires the instant their thumb comes off the hammer.
At least that's my recollection of single-action revolver operation. It's been a while.
And just for the heck of it, here's the version of The Rules of Firearms Handling that I was taught starting at age five:
1. Every gun is loaded until you KNOW otherwise. (Question: Is that gun loaded? If you don't KNOW, then the correct answer is "YES".)
2. Keep your gun always pointed in the safest possible direction.
3. Finger off the trigger until you are ready to shoot (and outside the trigger guard when handling the firearm).
4. Always be sure of your target and backstop (whatever is behind your target).
I don't know the modified protocols for filming a scene with firearms but obviously they were not followed. Presumably Rule #1 would become something like this: Every gun is loaded with LIVE ammunition until you KNOW otherwise.
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Not that it's necessarily applicable here, but just in case this is of any use to anybody: One of the most common scenarios for an accidental discharge is the "gun topic BS session with friends". You are with some buddies and the conversation turns to pistols and someone decides to show theirs. They remove the magazine and clear the chamber and show everybody that it's safe, everybody now knows it's safe, and you and your buddies handle it and admire the way it feels and (hopefully asking permission first) dry-fire it and nod approvingly at the nice smooth crisp trigger. Then the owner reloads the pistol but doen't put it away just yet (which he should have done IMMEDIATELY) and the BS session continues, perhaps drifting to other topics. Eventually somebody forgets that the pistol is now NOT SAFE and picks it up to appreciate that nice trigger action one more time and... let's just hope they had it pointed in the safest possible direction. So be maximum-alert perpetually vigilant if you are ever involved in a gun topic BS session where a firearm is handled. You may be the only one really paying attention.
