*** SPOILERS ***
A tremendous crock of shit masquerading as a horror movie phenomenon, Paranormal Activity literally has nothing up its sleeve beyond a few parlor tricks. Made on a shoestring budget of $15,000, one is still left wondering where all that money went, because it isn't on the screen. What is on the screen are two of the most boring people ever committed to high definition video occupying a million dollar four bedroom house that I'm not convinced they could even afford. (She's a student, he's a "day trader". Sure.) The house is too well-appointed to be creepy or claustrophobic and creates zero atmosphere for horror. Also in the house with them is a demon, or something, that's haunting the girl. So says a "ghost psychic" who, in one of many LOL moments (although this one was probably intentional), couldn't wait to get out of the house. The girl is told that leaving the house wouldn't help and that the demon would follow her, but it's never clear why the leaving the house option is never explored anyway. (Best guess: there was nothing in the budget for shooting exteriors.) The couple are haunted over the course of 20+ boring days and nights. The demon or whatever takes for-fucking-ever to pick up its game, going from spoooky things like turning on the hallway lights and loud stomping noises to setting fire to a Ouiji board to finally getting physical. The demon is out of the bottle when one quickly realizes the demon will only do as much as the budget will allow. After an interminable amount of stalling, Paranormal Activity empties its bag of tricks in the final minutes for a startlingly banal payoff. The fact that Paranormal Activity has made 100x its production budget at the box office is a testament to the ability of Paramount Pictures' marketing department to bamboozle moviegoers. They're laughing all the way to the bank.
For a similar idea to but a much better movie than Paranormal Activity, try The Strangers.