Thing is, neither knows what we all know, and both have humanlike reactions. Plus redshirts don’t always die immediately, just eventually. So you could get quite a long action sequence with redshirt attempting to dodge and getting more and more amazed he’s not dead yet, while stormtrooper eventually runs out of ammo and it becomes a mano á mano fight (I’m sure your brain can play you the correct music for this part) until redshirt dies.