The film was shot in just three weeks.
The film was originally going to be named "The Further."
Thirty-three violins were used for the theme music.
In 2015, there was an Insidious-themed maze at Universal Studios, Orlando for the annual event of Halloween Horror Nights. The maze was based on the three "Insidious" films released at the time. In 2024, there was another Insidious-themed haunted house at Halloween Horror Nights Orlando and Hollywood featuring all five movies.
James Wan, who directed both this film and Insidious: Chapter 2 (2013), and The Conjuring (2013) and The Conjuring 2 (2016), had The Conjuring in mind, while directing the first Insidious movie. A number of coherent points can be spotted in both the first and the second Insidious movies.
Bonnie Aarons: (at around 1h 20 mins) The actress, who plays the Nun in the Conjuring universe (also created by James Wan), has an uncredited cameo as the creepy 1950s styled mother who stands at the ironing board, and again sitting on the three seater lounge after being 'shot', and lastly standing in front of Josh alongside the equally creepy husband and daughters. The cameos appear in the scene where Josh Lambert astral projects into The Further to look for his son Dalton. He comes across the younger astral projecting version of himself who points out the previous house Josh, Renai and their kids lived in at the beginning of the movie.
James Wan: [puppet] (at around 30 mins) When Josh is in the classroom, on the chalkboard behind him, the name "James Wan" can be clearly seen, as well as a drawing of Billy, the puppet from Wan's Saw (2004), with the number "8" written under it. The rest of the names are the actual names of people that worked on the film.