I have watched many action animations and this question keeps bugging me. Does anyone know?
Stop-motion animation builders often have to resort to tricks to make the animation work. For example, they will use wires or other fasteners to attach models in certain positions in order to hold them in place while they move them. Many purists would avoid such techniques in a normal build but for animators there are larger overriding concerns. Cutting elements would certainly fit within the possibilities.
Also many builders feel free to modify any elements they have, including cutting them or painting them. How that sits with you is up to you.
You can just pull the torso and the legs of a minifigure apart, put a lego knife between the torso and the grooves of the legs and put it back together!
You could try cutting the knife with scissors or well, a knife.