During the first test screenings, people started leaving the theater. Originally the crew thought this was because the film wasn't going over very well with its audience, only to discover that people left the auditorium because they couldn't handle the intensity of the piece.
Paramount acquired the US rights for $350,000. The film went on to make $193 million worldwide, making this the second most profitable film ever made based on a return of investment. (First most profitable is The Blair Witch Project (1999) which cost $22,000 and made $240.5 Million)
The role of Dr. Johann Averies was cast with a real Paranormal Investigator from the Independent Investigations Group, Spencer Marks. The role was shot to help explain certain anomalies in the film. The role was predominantly mentioned in the movie, but the footage never used.
Paramount Studios utilized paranormal researcher, Christopher Chacon, who is recognized as one of the world's foremost experts on paranormal phenomena, to promote and publicize the film. Chacon also works in the entertainment industry as a writer, director and producer.
Survivor (2000) runner-up Katie Gallagher was originally cast to play the role of Katie, but was laid off because Oren Peli decided she was too "well-known" to keep the film as real as possible. Coincidentally, both Katie Gallagher and Katie Featherston have the same name as each other, and the main character.
(at around 4 mins) The guitar that Micah (Micah Sloat) plays towards the beginning of the film is a "trans blue" Line 6 Variax 700 six-string electric guitar. The guitar, created by amplifier manufacturer Line 6, was one of the first in a line of pickup-less electrics (meaning the electronics are internal) and features the ability to model or mimic "up to" 25 different stringed instruments including the distinct sounds of a Telecaster, Stratocaster and full hollow body electrics.
A third ending was planned where the possessed Katie corners Micah and bludgeons him to death with his precious camera, while viewers would watch from the camera's POV. In fact, this version of the ending was so complicated - not to mention brutal - that Peli never actually shot it.