If you don’t pull it back far enough to lock in place, the hammer will drop.
Ever see those westerns where they use the side of the hand to slap the hammer back over and over to get rounds out quick?
Single action means the trigger only does one thing - drop the hammer. You pull the hammer back and you hear a click, meaning it’s “locked” in place until you pull the trigger.
"If you don’t pull it back far enough to lock in place, the hammer will drop."
But unless the half cock sear is broken or worn, it will fall to the half cock position. The half cock is needed for loading because the Single Action Army (and reproductions) are loaded by putting the hammer on half cock which allows the cylinder to spin freely.
A Single Action Army on half cock:
