In just trying things out, I have put a knife in my teeth and clamped down while doing something with my hands and then picked the knife up again. I found no matter how easily they do it in the movies, it really freaking hurts the teeth and seems a completely silly and unrealistic approach to keep a knife handy while using your hands to climb something.
Is there an actual way to do this without hurting your teeth, or is it just a Hollywood thing that looks cool but is completely unrealistic?