The script started out after Zach Cregger read the Gavin de Becker's book "The Gift of Fear," which encourages women to trust their intuition when confronted by obviously dangerous men. He used it as a writing exercise and began crafting a thirty-minute short that consisted entirely of a conversation in which a woman continues to ignore a mounting series of red flags. He liked it well enough that he knew he had the makings of a longer film and began conceptualizing a broader story for the characters.
Richard Brake shared a story on Instagram about attending a late night screening of the movie, noting that a particular group of teenagers sitting a few rows behind him were really into the film and were terrified. When it was over, they were leaving the theater, using the flashlights on their phones, when the light accidentally caught Brake's face, the teens recognized him from the movie, then screamed and ran from the room in genuine fear.
Zach Cregger originally offered the role of AJ to Zac Efron. Efron turned it down, causing Cregger to rethink the role and offer it to Justin Long, who accepted.
Zach Cregger revealed that he had a hard time getting the movie made due to its very unconventional style and plot, with even A24, a studio with a reputation for producing artsy horror films, passing on it. Ultimately, 20th Century Studios decided to take a chance with it. The result was a sleeper hit that made its minuscule $4 million budget back about ten times, with positive reviews and even more of an audience on streaming.
The Hollywood Reporter exposé on AJ is written by Kim Masters, who is a real-life reporter at the trade publication. In the wake of the Harvey Weinstein cases, she became known for breaking stories about men in the film and TV industry who have been accused of sexual assault.