Imagine you are working all day at your office. You interact with so many people. Many people come to your office or desk uninvited. They just show up standing besides you, you at first fail to notice them, then suddenly you become aware of their presence. You travel on a subway or in a bus or shop in a department store, but when a totally stranger face or body shows up in your eyesight, you just take it granted - let alone stun or scare.
Now you come back from your office or bus or mall to your two bedroom apartment where you live alone. You go into the kitchen, start cooking... after few minutes you realize that a total stranger is standing 4 feet from you looking at what you are cooking.... you will be just short of a heart attack. However benign and harmless that strange face is, you will skip few heart beats before coming to senses. It is a regular stranger whose presence you wouldn't even acknowledge, had it been a grocery store and not your kitchen.
Then why is it so scary?
The answer is simple. We feel anxiety if something unexpected happens. And non-expectancy of someone other than you being in your apartment is extremely high "in your mind". Naturally your anxiety in response is also very high, almost a shock or horror.
This is what I think about the concept.
Imagine of a finitely practical situation in which one person's non-expectancy of an event is entirely commonplace occurrence for other person. And the situation should be practical in the sense that the audience should identify with both persons' points of view.
Imagine such a situation and you can have an intelligent horror movie.
I guess this is a general equation for horror movies, everyone just finds a different solution that satisfies all sides of it.