When someone attempts to be a mirror image of another person by standing behind a glass pane, sometimes only an empty frame or a gap in a row of mirrors, copying their movements exactly. Sometimes this is to hide the fact that they have broken the mirror, and they don't want the other character to find out. In Real Life, this is almost impossible to do convincingly. If you've ever done basic drama in school then you've probably done this exercise and know just how hard it is to synchronise your actions with those of another actor, even with proper forward planning.