Social interaction proceeds fluidly in everyday life

Social interaction proceeds fluidly in everyday life because people have shared definitions of and expectations about routine situations. As we go about our day, we generally assume that people are going to behave a certain way and observe certain norms. When they don't, we may sometimes know how to respond, because we have seen that kind of violation before. For example, if someone cuts in front of us in line, we probably already know how we would react to that breach of common courtesy.
But other norms are so ingrained and automatic that we can hardly imagine anyone breaking them. Because we have never been confronted with the behavior, we have no pre-set idea of how to react when someone fails to behave "normally." For instance, if you were sitting in a near-empty movie theater and a stranger suddenly sat down directly beside you, how would you react? Chances are, you have no idea because it's just not something people do.