What 2009's "Paranormal Activity" did for haunted house movies, "The Last Exorcism" hopes to do for devil possession. The results aren't quite as successful—and neither of them can hold a candle to 1999's "The Blair Witch Project," still the reigning champ of faux-documentary horror films—but director Daniel Stamm still exudes a forthright conviction for the material, cooking up a number of unsettling moments. If the first hurdle of a motion picture purporting to consist of "real" footage is that the acting be seamless, so natural the audience is never taken out of the illusion, then "The Last Exorcism" clears this jump without breaking a sweat. The slow, steady, but fully enthralling first half suggests great things to come, but it is in the latter section when the more conventional scare tactics get underway that mild disappointment sets in. For reasons that can only be construed as a creative miscalculation, Stamm feels the need to undercut the raw camera footage with a predictable, obvious music score. This introduction of artifice into what is intended to be authentic, untouched video recordings is woefully unnecessary, way too on-the-nose in telling the viewer how he or she is supposed to feel when silences and natural sounds would have made a deeper impact.

Reverend Cotton Marcus (Patrick Fabian) is a family man and all-around nice guy so beloved by his Baton Rouge, LA congregation that he can even spout off a recipe for banana nut bread in the middle of his sermon and receive hallelujahs in return. Quietly faced with self-doubts about his faith, Cotton has nonetheless continued with his profession, even travelling around and performing exorcisms for people who believe they are possessed by darker forces. Cotton sees the whole thing as a sham and has decided to take along a documentary crew to what he intends to be his last exorcism. The alleged sufferer is 17-year-old Nell Sweetzer (Ashley Bell), who lives on a secluded farm with father Louis (Louis Herthum) and brother Caleb (Caleb Jones). Their animals are being mysteriously slaughtered in the night and Louis is convinced Nell's body is being overtaken by...something. What starts out as a jokey outing for Cotton to prove the lack of merit in the idea of possession soon takes an unforeseen turn into the bizarre and outright unholy.

"The Last Exorcism" builds terrifically at the onset, properly introducing documentary subject Cotton Marcus and delving into his day-to-day life as a reverend. Even though he's a bit crooked in taking money from people for exorcisms he doesn't believe are true, it is established that his insurance doesn't cover his young disabled son's doctor bills—instant sympathy right there—and the services he provides, if a crock, still bring peace and solace to his believing customers. There is a sharp satirical bent to the first half—Cotton's tools used in his exorcisms include hidden audio devices full of beastly sounds and a crucifix that sprays out a poof of smoke from the back—and also some interesting comments about faith, Christianity, and personal belief systems.

Patrick Fabian (2005's "Must Love Dogs") is full of easy-going charisma as Reverend Cotton Marcus, the kind of entertaining character you want to follow and learn about. He has a sense of humor, yes, but he's also very much believable, rooted in reality. When things jump the rails into spooky territory, however, the film becomes more concerned with hitting its genre-laden traps than it is in continuing to be a character study. There is a particular missed opportunity in exploring the psychology of a man whose entire way of looking at the world and beyond must change when evidence points to demonic possession being a legitimate concept, after all. He continues to strive to help Nell, but the viewer misses being let in on his thought process. Looking like Michael Cera's twin sister, newcomer Ashley Bell is astonishing as Nell. Just as Linda Blair was in 1973's "The Exorcist" and Jennifer Carpenter was in 2005's "The Exorcism of Emily Rose," Bell is so sweet-natured and innocent in her early scenes that it is all the more disconcerting when the evil inside her shows itself. Credit must also go for steering clear of many of the tropes found in this sort of film; there are no spinning heads or pea-green vomit in sight, but there is a surprising amount of bloody carnage for a PG-13 rating. One supposes the MPAA thinks it is okay to show a cat ripped to shreds rather than a human.

The finale of "The Last Exorcism" is where the biggest problems come in. Although there is a wicked symmetry to the ending based upon scenes that came earlier, the overall twist comes way out of left field and doesn't quite escape from close scrutiny. It also is startlingly abrupt, the appearance of the end credits met with groans and an "is-that-all-there-is?" feeling. Director Daniel Stamm is fully capable of ratcheting up a sense of eerie disquiet and mounting tension, but why he feels the need to devalue the purity and commitment of his cast's performances with music and sound effects that scream of post-production tinkering is anyone's guess. Fortunately, this never reaches the overwhelming and annoying heights of 2008's "Diary of the Dead," a fellow POV horror effort that not only had an inappropriate score, but also horrible acting. "The Last Exorcism" is at least still effective, even in its flaws. It's a worthwhile effort, but one that has been overthought by its makers and suffers the consequences.