Many single action reolvers have a very light trigger pull where just resting your finger on the trigger can make it fire. Even if this were the case the hammer must be manually cocked back and that requires an intentional act.
I have a double action SW R8. It has about a 12 pound double action pull. When the hammer is cocked, it takes very little pull to drop the hammer.
I am not excusing Baldwin, or buying his story. Who the hell is handed a half cocked weapon without resetting the hammer? His story is a fantasy that he hopes will keep him out of jail.