I just want to clear up the "I didn't pull the trigger comment" by Alec Baldwin. The pistol he was using was copy of the Colt Single action Army. It's a very old style of pistol with no safety features, It is usually carried with the hammer down on an empty chamber, so you have to cock the revolver, to place a new round under the firing pin then pull the trigger. These are the pistols that could be fanned, Which is holding the trigger down and quickly moving the hammer back so it slams' forward over and over till the gun is empty. You can tell it's this kind of pistol by the firing pin on the hammer.
If you put the hammer down on a full chamber and you jar the hammer the shell it will fire (right into your foot)
So, it is possible to fire the gun by pulling the hammer not to the trigger lock position, but just before that point and releasing the hammer, so it slams forward and fires the shell.