When you pull the hammer back on a single action and your finger is on the trigger the gun will go off when you let go of the hammer. That is how fanning shooting works. Dumbass.
You don’t have to have your finger on the trigger. Take that single action put a round in it under the hammer chamber and pull the hammer back until it starts to resist at the half cocked sear then let go of that hammer. You better not be pointing it at anything important because that gun is going to fire without the trigger even being touched. Only modern guns have trigger actuated firing pin blocks. Take a 1865 colt and do the above procedure it will fire 8 out of 10 times