The actors were screened for their improvisational ability, and given a 35-page outline of the movie. All dialogue was improvised. The directors began rationing the actors' food to make them as hungry as their characters. They were not told when strange noises would be made, or when the tent would be attacked. Makes for some great acting.
Pay attention to Heather's screams at the end; according to the directors, she wasn't acting, and was freaking out for real.
Method Acting: The three actors had to remain in character most of the time, apart from brief breaks. They could only break character by using the codeword "taco".
Real-Life Relative: The waitress who asks about Blair High School is played by one of the director's sisters.
Reality Subtext: Josh frequently mentions hoping he doesn't miss work. In real life he had to leave filming to go to his job, which is why he disappears earlier than the other two.
The Red Stapler: Burkittsville, MD, a quiet little town of less than 200 people, was flooded with tourists after the film's release. Its residents were not happy.
Viral Marketing: Including a decently designed website that's still up as of October 2014. To the point where the three actors were listed as 'Missing, Presumed Dead' on IMDB. Heather Donahue's mother even received numerous sympathy cards from strangers who thought her daughter had actually gone missing.
Josh was originally scripted to be attracted to Heather, though she would be oblivious to this. It was dropped out of fear of seeming too cliche.
In the footage that was shot, it was actually Josh and Heather who argued the most, rather than Heather and Mike - as in the finished film. Most of the arguments with Josh were cut down, to avoid the Unfortunate Implications of both men ganging up on Heather.
The stuff shot in the motel room ran for ninety minutes, complete with the men reading poetry to each other and some drunken arguing.
An early idea was for the two fishermen to be revealed to have been playing a prank on the students the whole time.
Multiple ideas for the ending included a life size stick figure chasing the students, Mike being hanged or else found pinned to the wall like a stick figure.