Movies work by making us think that they're real. Most movies accomplish this by imitating life, so we forget that what we're seeing is illusion. When they scare us, it helps to remind ourselves that "it's only a movie." We know that someone is filming this in a studio somewhere, with scores of people on hand to make sure the illusion never fails. "Calm down. It's only a movie."

The Blair Witch Project is designed to keep us from saying that so easily. The premise is that three student filmmakers are making a documentary investigating the ghost stories of a small town. The first screen tells us that we are about to see their footage, recovered a year after their disappearance. The rest of the movie shows the filmmakers at work. The movie is entirely shot in grainy video and 16mm film, often in bad light or with bad sound, through jerky, rushed shots. There's no score and no opening credits. On the one hand, this makes it plausible that the movie is no illusion. It seems to be a student documentary made on the cheap that fully demonstrates the power of the storied Blair Witch. But because the movie never tries to create the illusion that we're seeing unfiltered reality, we know that we're sitting in a theatre, watching something. That bit of detachment keeps us aware that what we see is not real, even though it claims to be. It's only a movie.

That's not to say that The Blair Witch Project isn't deeply creepy. While most horror movies build to a few terrifying moments, The Blair Witch Project manages to sustain tension for minutes on end. By the end of the movie, even the pastoral daytime scenes are uneasy, and they get shorter and shorter, while the night scenes feel nerve-wrackingly long. Also, the scary things in most horror movies are outlandish and laughable even as they scare us. Scream makes a virtue of this, as it winks at every silly clich of the genre. This movie terrifies, however, with ordinary things-piles of rocks, bundles of twigs. Since similar objects appear in many shots, the menacing presence never seems to leave. And we can't laugh off our fears, so the tension doesn't dissipate.

The movie's premise prevents the directors (Daniel Myrick and Eduardo Sanchez) from playing many attention-grabbing cinematic tricks, so good acting is crucial here. Fortunately, all three principals give rounded, believable performances even while improvising much of the dialogue. Heather (Heather Donahue) plays the director and narrator of the documentary. Her drive keeps the project going, but her badgering of jockish cameraman Mike (Michael Williams) and easy-going soundman Josh (Joshua Leonard) causes tension. As things go awry, however, the power structure breaks down. Their relationships become more subtle and volatile as their fear wears on them and paranoia looms. They are each sympathetic, in their own ways, and our caring about them makes their plight all the more gripping.

The Blair Witch Project is unique and scary. The premise is original, and the movie taps into many primal fears (of the dark, of being alone, of the unknown) without being cheesy or obvious. But I don't think it quite lives up to the buzz around it. When I got out of the 1:45 showing on Thursday afternoon, already all the shows from 7:05 on had sold out. Part of the reason must be that Kendall Square Cinema is the only place in the Boston area that's showing The Blair Witch Project. But I think it's also because of the intriguing premise, which sounds better than it is. It promises to be real, and therefore terrifying. With every jerky shot, we remember that "it's a movie," so "it's only a movie." That may be for the best. The Blair Witch Project is still chilling.