i think epikshi is right, if you look at dead space, you'll find that a lot of the scariness comes from sounds, and if you check with ANY game developer, they will tell you the same thing.
i have some suggested things you could put in.
1. ATMOSPHERE
creepy voice overs a brilliant for this.
if you edit the effets on your voice, and have a sound zone with a voice saying "i'm behind you...i'm so close, i can almost touch you...ha ha ha ha!" that would SCARE THE CRA* out of me.
2. LIGHTING
these creepy voice overs will suck if you don't have the right lighting for them. try making the ambience level about 30 and have a flickering light in the dark, then make him say it.
3. EFFECTS
try using sounds, dynamic entities and possibly videos to keep the player on edge.
for example: footsteps when the player is walking along a dark corridor, followed by a tinkle of shattered glass, then a sound.. was it a growl? no, it can't be... that's stupid, then, as the player is leaving the corridor, a bounding, a snarl, the player turns round... and BAM! there's nothing but empty space and the wind going through the corridor.
see what i'm getting at? with the right use of these 3 concepts any game-maker can make a horror game so creepy, so terrifying... you know the rest.
strike force delta (on the rails) http://forum.thegamecreators.com/?m=forum_view&t=152412&b=25
darkest depths http://forum.thegamecreators.com/?m=forum_view&t=151509&b=25