In psychological thriller "The Last Exorcism," a skeptic's faith is testedS

Before you see The Last Exorcism this weekend, forget everything you saw in the trailers. This is not a gorefest. It's a tense, character-driven story of skepticism and faith that will leave you utterly shaken.

Like many recent horror films, The Last Exorcism is a fake documentary. But unlike Paranormal Activity, or even Blair Witch Project, the motivations behind the documentary take center stage and immediately help us sympathize with the main character, Cotton Marcus (TV veteran Patrick Fabian, who is simply terrific). Cotton has been a charismatic preacher in Louisiana since he was a kid, working with his father in tiny local churches, and performing hundreds of exorcisms for believers. But he has a crisis of faith when his son falls ill, and he realizes that medicine - not God - has saved his child's life. On top of that, he's upset by a widely-publicized case of a young, autistic boy who dies during an exorcism.

In psychological thriller "The Last Exorcism," a skeptic's faith is testedS

Cotton comes to believe he's doing more harm than good with his exorcisms, and sets out to show the world how fake these rituals really are. That's why he wants the documentary crew along. He's going to show them all the tricks he uses to fool people into thinking he's wrestling with the Devil, when in fact he's just got a hidden speaker system howling from the walls and a crucifix rigged to squirt smoke when he presses a button.

We learn all of this in a series of short, fascinating interviews as the film opens. Cotton's obvious care for his son, plus his new faith in the power of skepticism to save people from harming their own kids, makes him seem like an ordinary man who just wants to do good in the world. Usually you can't wait for people to die in horror movies because they're such loathsome twits. But Cotton is a complicated, thoughtful person whose struggles you want to know more about.

Out of a vast pile of letters begging for his help exoricising evil spirits, Cotton pulls a random envelope, declaring that this will be his last exorcism. He barely skims the letter, from a man named Louis Sweetzer, before accepting the job. We follow him deep into rural Louisiana, where Cotton says poverty and lack of education are the fuel for faith.

When the crew arrives at the home of the Sweetzers, they're plunged into a horrific situation they're not prepared for. First of all, it turns out the possessed person isn't Louis, as Cotton believed, but his daughter Nell. Cotton is unhappy about working with a child, but he decides to go along with the charade, conducting an exorcism that the documentarians have intercut with little MAKE magazine-esque how-to lessons on building the smoking crucifix and setting up hidden wires to make pictures on the wall appear to shake. But the more he and the crew get to know Nell, the more obvious it becomes to them that she's suffering horribly - not from demons, but from schizophrenia.

In psychological thriller "The Last Exorcism," a skeptic's faith is testedS

Trying to protect Nell from her superstitious father, Cotton sheds his exorcist persona and does his best to intervene, begging Louis and Nell's brother Caleb to take Nell to a doctor. But her father refuses, and of course the fake exoricism has done nothing to help her: Each night, Nell continues to be "possessed," contorting her body into weird shapes and wandering around killing farm animals. In one brilliant scene, she even steals the camera and uses it as a weapon.

Her violent schizophrenia is far more terrifying than possession - especially because we feel acutely Cotton's empathy for her, his desire to get her the help he thinks she needs. And the more Cotton discovers about Nell's creepy relationship with her father, as well as the family's relationship with the kindly local minister - who says the Sweetzers left his church "because it wasn't medieval enough" - the more terrified we are that something awful will happen if Nell doesn't get medical help.

In psychological thriller "The Last Exorcism," a skeptic's faith is testedS

Of course there's always the possibility that Louis might be right about demons, which is something the born-again skeptic Cotton can't admit.

I cannot emphasize enough how well-acted and gripping this film is. It may be marketed as a horror film, but it has the pacing and emotional intensity of a thriller. The scenes of Nell's "possession" are perfectly poised in the sweet spot of total ambiguity. Is she possessed, as Louis believes, or is she a schizophrenic with violent tendencies? Until the film's harrowing ending, we don't know - and even once we reach the climactic scene, we're still left with questions. Not the bad, "that made no sense" questions, but the good kind, where we're forced to ask whether faith - be it the faith skeptics have in science, or religious faith - prevents us from helping the vulnerable when they need it most.