First things first, I hate the word "mockumentary." In fact, let that be the only time the word is mentioned forthwith. The problem is that the term implies connotations of humor and parody (This is Spinal Tap, Waiting for Guffman, et al), which gives other and especially more recent films of this style (Faux Amateurism?) a very debilitating handicap right out the gate. Luckily, The Blair Witch Project came out like a bolt from the blue and breathed fresh life into the style (Diegetic Archive Footage?) by applying its stylistic principles to the horror genre, where it's made itself surprisingly at home with works such as:
The Last Broadcast
The Poughkeepsie Tapes
The Blair Witch Project
Amateur Porn Star Killer
The Fourth Kind
As a filmmaker, I'm naturally attracted to the concept; I appreciate its grassroots, guerilla sensibility married perfectly to lofty goals and a wide vision. At the same time, it's got this fragility, a kind of precariously teetering insecurity best analogized to The Emperor's New Clothes. Basically, once you acknowledge the "realism" as "frugality," disbelief becomes unsuspended and suddenly you realize that you're watching a talented filmmaker stretch a budget in ways that seem more underhanded than clever. Whichever intention rings true, it's being given more attention than necessary. If you can manage to avoid the pessimistic outlook of "cheap and dirty," the outlook we're left with is that it's just another way to tell a story, like first vs. third-person perspective in a novel.
To that end, the best way to think of these films (whatever name we decide upon for them) is as the visual equivalent of the epistolary novel. It has all the strengths: a more relatable perspective, a more naturalistic approach to characterization, and a heightened sense of realism. Sadly, while it may not exactly have all the weaknesses as well, it's got a whole new set of them to overcome.
The biggest point of contention that's probably going to arise during a viewing of Paranormal Activity is one of scale and subtlety.
This leads to the inevitable comparison to The Blair Witch Project. Yes, it seems unfair to hold every "Found, First-Person Film" to Blair Witch, but that's because there's a lot of things that Blair Witch does right, and a lot to admire as well. Chief among these is its ambiguity. While the film ultimately speaks for itself, so to, er... speak, it really doesn't have a lot to say. Is it a witch? Is it a cult? Is it an elaborate hoax? Is it a conspiracy to commit murder motivated by a sexual power struggle? The point is, it's open to interpretation, and we're really not sure just what it is we've seen.
With Paranormal Activity, there's no question about what's happening. I won't spoil it, but I'll tell you the exact moment in the film when several things happen, among them getting a clear and solid sense of this movie's "reality," and myself nearly bursting out laughing, ready to dismiss the film as cheap and gimmicky.
The scene in two words: Ouija Board.
That'll be the extent of the description, but you'll know the moment when you come to it, and hopefully you'll see what I mean when I'm talking about subtlety and a sense of scope. At first, I honestly found myself fighting back a laugh, along with an overwhelming sense of disappointment. It's my own fault for expecting ambiguity and instead getting something that very clearly and blatantly spells out just what type of horror movie this is. It is not, in fact, a psychological thriller as these "Discovered Video Diaries (Hmm, DVD...?)" might lend themselves to, but something more straightforward. Don't let that deter you; again, we won't spoil it, but just because we know what is causing the hauntings doesn't make it any less effective.
To sum up, the best way to describe just what kind of horror movie Paranormal Activity is in relation to other horror films you might have seen is this: on one end of the spectrum is Poltergeist with The Blair Witch Project on the other, and Paranormal Activity is exactly halfway between them.
NOTE: If you rent this movie on disc, you're presented with two menu options at the start, one being to see the film with its original, theatrical ending, the other to see an alternative ending, which, in the film's complex production and distribution history, is more akin to the director's original cut of the film. While you're probably going to watch both endings anyway, I strongly recommend seeing the theatrical cut first before checking out the alternative ending.