Here's one of the reasons I know this movie is a success. After the flick ended, and as the audience filed out into the lobby, the young woman behind me said to her boyfriend:
"I have to go home and research what happened to Katie. Do you remember her last name?"
She thought Paranormal Activity was real. Or at least some aspects of it was real. Enough so that she cared about what happened to the character.
I've heard this movie be compared to The Blair Witch Project, based mostly in how the perspective is from the camera and the film is designed to look like a true story. But it's an unfair comparison.
The difference, to me at least, is that the audience watches the most intimate moments of a couple's lives during a 21 day period. We see them sleep together, play together, joke with each other and be frighted together. We see the couple fight, and we see them make up again. It's the intimacy that allows us to be frighted right along with them.
Quickly into the picture, we care about them; they're family.
That this movie is able to be (sort of) scary without blood, special effects or obvious monsters is a testament to how director Oren Peli develops the relationship between the audience and the characters on screen. Although I don't think the film is very scary, I do think it's more than entertaining.
And it's remarkable that's accomplished in the most minimal of ways.