Which elements of horror actually scare you?

[Ford Prefect]

Honestly, I'd have to say people. Smart, dangerous people especially so. Or just people. Screw zombies - Resident Evil 4 shows the way. I might have killed them in their hundreds, but the Ganados were downright creepy; especially the earlier villagers. Everyone of them is a person; that's why I found the infected from 28 Days Later to be effective as well.

Though it's a mix as well; I always found Event Horizon to be one of the more disturbing movies I've seen. For different reasons than the venerable Brother-Captain; I found it freaky mostly because it dealt with things out of the characters' pasts.

[DPDarkPrimus]

For me, I think realism is scarier than anything else. I find Nightmare on Elm Street to be the scariest movie ever. Largely because it seems possible. ....You watch the movie, and when you see Kreuger in your dreams that night, you are terrified, because you can't distinguish the movie from reality or your dream from reality. Any time that line can be crossed like that I think it's completely terrifying.

Realism is a big part of why TCM was so horrifying. It feels like a damn snuff film.

[Camel]

Movies do not scare me. The plot might draw me into it, and make me wonder, and care what happens. My rational mind is always telling me that zombies are fake. Vampires don't exist. There is no giant spider over there.

People cause that jolt of adrenaline. Territorial bullshit. Unfortunate confrontations.
Before, while, and after you deal with crazy people, you are on guard to act. People like that are like animals. You talk in a controlled, calm voice. Immediately disarm bullshit and direct attention away from anything that could lead to action. Tell them what they want to hear but direct attention to what you want them to focus on. Just like dogs.

I'm rambling. People actually scare me sometimes. Movies don't at all really.

[Sofia]

One thing I'd like to emphasize about the OP is that I'm interested in whether or not there's a connection between people's interest in certain elements of normal fiction and in similar elements in horror. I tend to be most interested in settings in both normal fiction and in horror, but I don't know whether or not that's simply a coincidence.

I've found that's true of me. Crazy people or faces that are just slightly deformed scare me the most, and I'm definitely most interested in character motivation and development in the normal fiction I read.

[Akhlut]

People, I'd think. Jack Nicholson in The Shining scared me more than most things.

[Dooey Jo]

Ghosts are probably the only things I find scary in fiction. And I don't mean all of them, just the really spooky spooks. I don't know why. Maybe because they are there somewhere and you know it and you know that they might do something at any time but you don't know when or what and you can't do anything about it. The scariest dreams I've had are the ones where I'm lying in bed and realise that there's a ghost in the room. Or the one where I took a picture of an empty road and then when I looked at the picture there were two ghostly figures there walking on the road towards the house, which made me realise that they must now be right outside the door! *shudders*

Not very many movies manage to get it right though...

[Asst. Asst. Lt. Cmdr. Smi]

I'd say people. Because I'm watching, and anything that happens to them I imagine happening to me, and the thought of that creeps me out.

[Grand Moff Yenchin]

I think I'd say places as well. Besides being the most unpredictable of the options, sometimes it seems that you really don't have much choice. Which makes things even worse. I wasn't scared of the Silent Hill spooks, but the thought that you're stuck in this limbo-esque environment for probably eternity really gets me disturbed and uneasy.

[Guardsman Bass]

Surreal environments scare me the most - things like Event Horizon.