Bang! But it's the shooter who's shot!

The villain stands over the hero with his gun pointed menacingly at him. We cut to the hero who has closed his eyes waiting for the inevitable, there is a bang - but wait! The hero is okay?? He opens his eyes to see the look of surprise on the villains face as he crumples to the floor, and behind him is SOMEONE ELSE with a smoking gun - they actually fired the shot and killed the villain!