Towards the beginning of the film, Julie takes a picture of young Katie standing in the driveway shortly before a group picture is taken. The photo being taken appears in both Paranormal Activity and Paranormal Activity 2 as an indication that the demon has started haunting them.
The symbols that appear behind the pictures in Grandma Lois's house are simplified versions of the Pentagram of Solomon (inverted) and the Triangle of Solomon, symbols depicted in the "Lesser Key of Solomon" and used to summon and constrain demons in classical Goethic Theurgy.
As with Paranormal Activity (2007) and Paranormal Activity 2 (2010), Paramount Studios once again utilized paranormal investigator/researcher, Christopher Chacon, who is recognized as one of the world's foremost authorities on paranormal phenomena, to internationally promote and publicize this third installment. Chacon also works in the entertainment industry as a writer, director and producer.
The trivia items below may give away important plot points.
In the static at the end of the movie, there is a frame that shows Dennis' and Julie's bed on fire. This is presumably the start of the house fire that is referred to in both Paranormal Activity (2007) and Paranormal Activity 2 (2010).
The film takes place in September 1988, with certain dates highlighted graphically. One highlighted date is 9/10/88, but then the events skip ahead past 9/11, a storytelling decision that was very important to the filmmakers. "Out of respect," Joost said. "Never gonna do that."
The camera seen on screen during most of the film is actually a tiny Hi-Def camera hidden inside the body of a VHS camcorder; for the first few weeks of shooting Smith did not have a viewfinder to see what his footage would look like.