Follow, follow, follow, follow, follow... basic continuity from scene to scene, because if you don't, somebody's going to notice. Everyone's favorite childhood movie was a nightmare shoot for the actors. Filming went on for sixteen hours a day, six days a week, for an entire six months. It was exhausting and didn't give the performers time to do much else with their lives. Get a haircut, for example.
Judy Garland's star turn as Dorothy made her megastar and endeared her to the hearts of millions, but there was something very strange going on with her hair. It changes dramatically in length from scene to scene. Would it really have been too much to arrange a haircut for her so things could stay consistent? In fact, isn't there literally an entire hair and makeup department on a movie set for this exact reason?