The shots in a film scene are usually not recorded in the exact order they will appear in the scene. Shots from one camera angle are recorded, then the camera is moved to another camera angle. Sometimes shots which appear back-to-back in the finished film are actually recorded on different days. Continuity from shot to shot is an important aspect of film making. Filmmakers must make sure that the actors’ makeup and wardrobe, as well as all of the scenery and props remain the same from shot to shot.
One of my students sent me a link to this video following a discussion of continuity. I think it makes the point pretty well.