I would suspect since they were filming a western movie the gun was a revolver.
If it was a gun from that era it would be single action which means you have to pull the hammer back with your thumb to cock it. Newer revolvers can be single and double action.
Double action pulling the trigger will cock and release the hammer. As far as I know almost all firearms that have an external hammer will have a "Half Cocked" position for the hammer
where the hammer is allowed to move part way forward and in doing so locks the trigger and the gun can't be fired until the hammer is pulled all the way back again.
If it was a gun from that era it would be single action which means you have to pull the hammer back with your thumb to cock it. Newer revolvers can be single and double action.
Double action pulling the trigger will cock and release the hammer. As far as I know almost all firearms that have an external hammer will have a "Half Cocked" position for the hammer
where the hammer is allowed to move part way forward and in doing so locks the trigger and the gun can't be fired until the hammer is pulled all the way back again.