What keeps Paranormal Activity from being quite as good as its obvious inspiration is a shortage of subtext. Blair Witch delivered plenty of shocks'n'scares, but what made it capital-A Art was its savvy analysis of mediation. "Nobody gets lost in America," Heather said, but the camera-obsessed characters find themselves more lost the more they look, and the final shot's multileveled attack on the act of viewing pulled the thematic threads together with sharp clarity.
Paranormal Activity doesn't go that deep, largely because its whole story is its central couple, and they're pretty generic---he's a cocky guy, she's a meek girl, and together they're just like every other couple we've ever seen in a horror movie. Some of the commenters at Jim Emerson's Scanners have actually had the most interesting points to make about the movie's themes----Jeffrey Simons noted the ways voyeurism becomes an element within the story, not just the way the story is told, and "Joseph" made the interesting suggestion that the whole movie is closer to Repulsion than Rosemary's Baby, a story about a woman lashing out rather than a woman persecuted.
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One aspect I haven't seen mentioned, though, is how the film plays with its own dog-that-didn't-bark: the young couple in the big new house... with no talk of children. But that's not to say children aren't a presence---at the other end of the hall from the bedroom there's a room we glimpse only in passing, with a big bed, and a giant teddy bear. That seems to be where the demon comes from in many of the scenes, and the stuffed animal suggests that it's the planned bedroom for the planned child, should Micah and Katie ever start filling this big house with something other than expensive toys.
More explicit, but more witty, is the way the couple's dynamics get affected by Katie's haunting---they start out affectionate, though a little out of joint with each other. But as the hauntings get worse, sleep deprivation becomes the defining element of their relationship, and much of the "negative energy" that feeds the demon is generated by their snapping under the pressures of exhaustion, like many a young couple with a new visitor making their nights into constant vigilance. Similarly, a major turning point of the film into full-on horror is when an invisible presence crawls into bed with them, as though the demon itself is a nightmare-prone toddler determined to enact Oedipal rage.
A terror at sexuality floats all through the movie, and not just because the locus of its horror is the couple's bedroom. Katie's haunting began with an apparition at her bed when she was eight, and her telling of the story, complete with helpless sister, could easily be read as a recovered memory of molestation. The signs of the demon---breathing on the neck, invasion of the bed, and grabbing of the leg---resemble the moves of an aggressive seducer (particularly when we see that the demon's ultimate goal is to be inside Katie). And every time Micah brags about setting up a camera in the bedroom, it's impossible not to think that he's planning to make an awesome sex tape once this whole demon thing blows over.
What keeps Paranormal Activity from greatness is that it never quite gets specific enough with any of this---the themes float through the film but don't really develop, and it's frequently disrupted by superfluous elements, like a completely pointless bloody cross (in a movie where no character shows the slightest religious inclination---Blatty wept!). Still, the material's charged, the directing is solid----the slightly dutch-angled shot of the bedroom, with the door and the hallway beyond it balanced by the visual weight of the bed, is an image film students should study carefully---and it's ultimately pretty goddamn scary. A good spook-show rather than great cinema, but still enough tension that I'll remember it for quite a while. And if nothing else, I'm still overjoyed that it whupped Saw VI.