Just take a look at that weekend box office. Sure, the critically panned Couples Retreat came in at #1, earning over $32 million on 3000 screens. But scan down the list and look at #4, which was Paranormal Activity. It earned $7.9 million on 160 screens. That's not a typo. One hundred and sixty screens. If we take the average, Paranormal Activity earned $49,375 per screen, and Couples Retreat took in a paltry $10,666 per screen. That's five times as many butts in the seats for the horror film than for the unfunny comedy (which means that there must have been a lot of empty seats at the latter). There's a simple reason for this: Paranormal Activity is a genuinely scary movie.
The same goes for any of the "body genres," i.e. comedies, steamy films, weepies, etc. If they genuinely work, and genuinely elicit the response that they promise, they will be a hit every time. Horror buffs -- myself included -- probably see more than a dozen new "scary" movies in the theater each year, but it's only once every few years that we actually get scared at one of them. Paranormal Activity achieves this by doing something very simple and not at all new: it doesn't show anything (or, rather, it shows very little). It knows that nothing that can be shown onscreen can equal the fears and nightmares of the people in the audience, and that the fear of the unknown is the greatest fear of all.
Val Lewton was one of the finest practitioners of this theory with his series of nine great films made for RKO between 1942 and 1946. In Cat People (1942), a woman takes a night swim in an indoor pool; the ripples of the water are reflected on the walls all around. We hear growling from offscreen, but we can't see it; the woman can't get out of the pool because the sound seems to be coming from all over. That's much better than getting to see an actual killer cat. Other famous films that managed minimalist scares include The Haunting (1963) and -- like it or not -- The Blair Witch Project (1999). (I have to admit to being scared out of my wits at certain David Lynch movies as well.)
The recent Sorority Row (347 screens) had one such good scene, after someone dumps some bubble bath into the sorority house's outdoor hot tub. Over the course of a big party, the suds slowly seep out of the tub, grow and tumble across the entire yard. In one tense scene, a girl goes poking around for the killer in the waist-deep suds, and he could be anywhere. The scene ends, of course, with a visual of death by flare gun, but it's one inspired moment in an otherwise uninspired film. The Final Destination (365 screens), while kind of fun, is the exact opposite of something scary; absolutely everything is shown in this film, and the only suspense lies in watching the various intricate deathtraps unfold.
It almost makes you want to define a new subcategory of horror films, something special for the really, truly scary ones, although like the word H.P. Lovecraft frequently used, perhaps it's best just to call them "unnamable."