The script started out after Zach Cregger read 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 that 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 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, along with positive reviews and an even wider audience on streaming services.
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.
While the exterior neighborhood shots were filmed in the Brightmoor neighborhood of Detroit, the rest of the film was shot in Sofia, Bulgaria.