Paranormal Activity was an independent film that Paramount Pictures decided to distribute.
Because Paramount got involved, some of the original film was altered (the ending and placement of certain shots in the film's timeline)
Steven Spielberg himself suggested the ending you see in theaters.
Micah Sloat decides to videotape some of the nightly activity in his house. His girlfriend, Katie Featherston, isn't as excited. She claims that ever since she was 8, there's been some sort of presence following her.
The video you see is the tape authorities found inside the house.
The stuff during the daytime is great. It's funny and entertaining; just like watching anyone's home movie.
Then they get into bed. In fact, this plays to your psyche. Whenever you see them in bed, with the night vision turned on, your stomach turns over because you know something is about to happen.
You get a heavy dose of thuds and screams, add in some powdered footprints and a psychic who is too afraid to stay in the house.
This movie plays to your basic fear. If it was some alien monster, this wouldn't have been as scary. But because there's always a sliver of question concerning the existence of demons, what you don't see frightens the scream out of you.
I came home with my ears perked for any little sound. Paranormal Activity is a solid "homevideo" scare.