lostinidlewonder, how do you feel about actually playing a passage for a student? Many teachers think it's a bad idea, because the student should be learning how to play in their own style, not just copying the teacher.
1. Yes, it's true, that the difference between good and bad is often very subtle. Only a single note a slight bit too loud or too late or too short - and the whole whole phrase is ruined.2. Because of 1. it's almost impossible to correct such things just by correcting movements. The control of the playing has to do with the preliminary imagination of the sound and the listening to what your playing really sounds like afterwards. Correction of movements can be helpful in some cases, but that's only a very small part of the work. Ear training is the biggest part.
Sometimes the bad sound can be from bad technique,
Bad technique? What's that?