You're watching a horror movie.
The characters are going about their lives, blissfully unaware that something horrifying is about to happen. You the viewer know that things are going to end badly, though, because you know it's a horror movie.
Someone opens a closet - a bloody corpse could fall out!Or they're drinking a glass of water - which could be infected with a virus! Or they're talking to some guy - who's probably a serial killer!And so on.
The effect of this - and a good director can get a lot of mileage from it - is that scenes which would otherwise be entirely mundane, are experienced as scary, purely because you know that something scary is going to happen, so you see potential horror in every innocent little thing.
An expectation as to what's going to happen, leads to you interpreting events in a certain way, and this ...
