little stain: Thanks, it really runs smoother. But, as I wrote, I wanted physics in order to interact with other objects.
Yes, I do know about the image editor and the ability to set different collision polygons. But thats part of the problem. As I wrote before: "Everything fine, but my problem is the changing collision mask. E.G. from square to circle. I tried to connect the character-change with a change to a different animation frame of the placeholder which has the circle collision-mask applied. That doesn't work, the character doesn't change and freezes."
The change from square to circle (with the change of the collision masks) results in the game freezes.
Furthermore: If my characters don't have physics on, they remain straight and don't take the angle of e.g. a ramp they're stepping on. As in your project-file. Check by yourself.
RandomOutput: Thanks for the "if playing..."-tip.