There was a brief misconception online that the movie was to be released in four different parts after Neon released four characters posters titled "Part 1, Part 2, Part 3, and Part 4."
Jessica Henwick actually took ASL lessons to prepare for her role as Beth, a mother to the mute child Alma.
The film is strongly influenced by the works of Dario Argento and Italian horror films known as "Giallo", particularly the films Suspiria (1977), and Phenomena (1985). Other films that this one took visual inspiration from include Halloween (1978), Dressed to Kill (1980) and Lost Highway (1997), which Tilman Singer credits as his favorite films.
John Malkovich and Gemma Chan were initially cast for the roles of Henry and Beth before they dropped out due to scheduling conflicts. Sofia Boutella was also cast in an undisclosed role, but dropped out before shooting.
Originally scheduled to release on May 3, but it was delayed to August 9, for undisclosed reasons.