[Tupelo]

RE: “If he didn't pull it back enough to engage the trigger, it was the same as pulling the trigger.”
Probably

Or if he unintentionally squeezed the trigger as he drew the gun.
Possibly

Or if the sear was badly worn
Possible, but highly unlikely

[Chad C. Mulligan]

Both original SAAs and early Pietta replicas have a half-cock notch on the hammer. If the hammer is dropped without the trigger held back the hammer will stop on the half-cock notch. Later Pietta replicas have a transfer bar added to the mechanism.

Bottom line: No matter what he has convinced himself of, the trigger HAD to be pulled for the gun to fire.

[mad_as_he$$]

Not to get too far afield but “Tex” Grubner shooting him self is one of my favorite vids:

https://www.youtube.com/watch?v=p3kJ6SU3ycs
Trigger fingers can be rouge elements in the situation.

[Dusty Road]

“Or if the sear was badly worn
Possible, but highly unlikely”

I believe the SO is still holding the weapon as evidence so that would be easy to determine.

[Iceclimber58]

I am no expert.
Every modern single action revolver I own has a transfer bar between the hammer and firing pin. The trigger must be pulled for the transfer bar to let hammer strike the firing pin.
Was the gun he was using so old it had no transfer bar?

[gundog]

This poor gun has likely been abused by cast and crew fanning it. Free gun, free ammo...people will abuse it. If it was being used for target practice and screwing around, and not properly naintained, who knows?