I think that the reason most people don't stutter when they sing, mock, imitate, act, ect. is because they get a chance to feel like something else. When we stammer, it is usually the fear of stammering that actually makes us stammer. I think we stammer because we are afraid to show ourselves. When we act we don't stammer because we are not ourselves - we are playing a role as another person. My opinion, works for me :)