Games already seem like dream states. You're wandering around a strange new world, where you simultaneously are and aren't yourself. This is already an inherently uncanny experience. That's why a well-made horror game feels so claustrophobically like being locked inside a really bad -- by which I mean a really good -- nightmare.
Still, there are some interesting limitations on the form. I find that scary games almost always lose their scariness after about three hours. This is due to the inherent repetitiveness of games: After you've fought your 200th "splicer" in BioShock, you're pretty accustomed to their gurgly ramblings, their patterns of attack, the boo-yah outta-nowhere teleportations. I was still tense, but no longer, you know, wetting myself.
The only way a game can continue to frighten you is if it constantly subjects you to new scary things, keeping you eternally off balance. But few publishers are willing to spend money on enough designer hours to churn out 40 hours of genuinely new content. Instead, they inevitably wind up recycling the same opponents, the same animations, and perhaps worst of all, the same audio cues. (The quickest way to ruin a scary mood is to have the monsters endlessly repeat the same two or three catchphrases over and over again; it begins to feel like telemarketing.) The best horror games -- I'd include some of the Silent Hill and Resident Evil titles in this category -- have come the closest to keeping things fresh as you play.
Still, I'm not complaining too much. BioShock was plenty freaky enough; I wrote this column with the lights on.
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Clive Thompson is a contributing writer for The New York Times Magazine and a regular contributor to Wired and New York magazines. Look for more of Clive's observations on his blog, collision detection.