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're frightening, it is comforting to remember that "it's only a movie." 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 producing a documentary investigating the ghost stories of a small town. The movie begins by explaining that their footage has been recovered since their disappearance a year ago. 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. 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 only 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. Scream makes a virtue of this, winking at every silly cliche of the genre. But Blair Witch invests ordinary things--piles of rocks, bundles of twigs--with horrifying meaning. Since similar objects appear in many shots, there's often a split-second when we're reminded of the unseen menace. 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, camera-toting Mike (Michael Williams) and easy-going sound engineer 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 grows. They are each sympathetic in their own ways, and that 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 it doesn't quite live 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. The movie promises to be real, and therefore terrifying. With every jerky shot, we remember that "it's a movie," so "it's only a movie." Good thing.