Dum dum dum da da dum dum...
Horror based games are all about putting the player under pressure. No, scratch that... all games should be about putting the player under pressure. A game where the player doesn't feel the heat is no game at all in my humble opinion.
The trick is to ensure there's enough gaps in the pressure to make it an enjoyable experience.
One of my favorite moments from Half-Life 1 was the "surface tension" level with the Apache helecopter where the player has no choice but to run away. Sure the first time you come across it you may chance a few shots with your pistol but you soon get the message. RUN BITCH!
I love the reversal in circumstance. Here's our bold player churning his way through every foe the game throws at him and then suddenly you're in some serious trouble.
It leads to some real action moments. Who can forget the amazing "frying pan or fire" decision with the chopper closing in behind you and you make the decision to jump off the dam. Only then do you discover that the water below is infested with big bad fishes!
Trying to replicate this kind of experience in other games is really quite difficult. The huge array of enemies are all designed to be killed. Hell, in Left 4 Dead even the tank can be brought down with pistols if you're team is coordinated enough.
I'm tinkering with this idea at the moment. A great way to ramp up the pressure is to show the player what they are going to have to go through before they have to go through it.
For example, the players have to descend into a maze of underground corridors which are for some reason infected free. They traverse several difficult areas and finally reach their goal, a power switch needed to activate the door they came across 5 mins ago. It's about 3 levels down in a dank, dark utility room.
They then hit a switch and the lights come on which, in turn calls a panic event.
They'll then have to fight their way back through all the difficult areas in order to proceed.
The player knows what's coming, they know it's going to be a nightmare but push on anyway because the areas are familiar.
Watching the clown with a knife between it's teeth grinning up at you then entering the building below you has always been a recurring nightmare of mine!
That's horror. The knowledge that a terrifying foe is on it's way and there's nothing you can do but face it.
Another idea I've always wanted to try out but never been able to establish a decent map layout for is having a team deathmatch or CTF map where both teams spawn next to each other, seperated by glass or forcefield. Both teams get a good look at each other and know what's coming. Plus it's fun to make rude gestures through the window before a match kicks off!
Anyway... I digress...
Forewarned is forearmed. Give the player a hint of what's in store and you can be sure they'll be pooping themselves before it arrives. You can even play with this a little and have the arrival of the foe be something completely different to what was expected.
Now lets all go and make someone defecate!