Never say "who's there?" Don't you watch scary movies? It's a death wish. You might as well come out to investigate a strange noise or something.
- Ghostface, Scream (1996)
It's the old horror movie cliché: why do people say 'hello?' when the killer's creeping around their house? People have been laughing about this cliché for so long, it's spawned memes...
Let's take a look at the real psychological and scientific reasons for this 'hello' phenomenon...
A natural response to 'uncertain' stimuli, your brain is determined to categorize this event as 'normal.' By setting up a 'normal' question -- 'Hello?' -- you test your environment, setting up a 'safe' answer (i.e. your housemate grabbing a drink). There's also the simpler answer: we just like to make little comforting noises to ourselves when we're scared. All of us can remember muttering things like 'It's OK' under our breath at some point...
Is It Just Bad Writing?
This theory posits that people in movies say 'hello?' at the wrong moment just because screenwriters are hacks who all copy each other. If movies don't even try to imitate reality, do real world rules -- such as psychological principles -- even apply to them?
Someone Harmless WOULD Answer
Let's think of the possibilities that involve someone in your kitchen who doesn't wanna stab you to death: a drunk housemate stumbling about or a concerned neighbor who wants to tell you that you left your door open would answer you, right? So saying 'hello' is kind of sensible... it's relaxing when you get NO answer that's not!
It's Actually Not So Weird
As viewers, we know we're watching a horror movie, we know who's a main character and who's expendable, and we are given clues like foreboding music to tell us what's going to happen shortly. The character themselves, in their own 'real world' knows none of this; nobody goes through their own life thinking, 'I'm not a main character, better call the police over the slightest noise in my kitchen'!