How did they film the cutting scenes in the movie thirteen?

I'm not 100% sure this is correct, since I wasn't in the movie. BUT I was in theater for a long time and what we did when there was a scene like that was put theatrical prosthetic "skin" on the persons arm to make it look like wasnt even there then make a cut in it and fill it with fake blood. That way when the person was cut, it would cut the fake skin and blood realistic looking blood.