Contrary to the marketing's claim that the film was shot in one uninterrupted take, the entire movie was actually shot to mimic one continuous real-time take, with no cuts from start to finish, as a result the time span of the film's plot is exactly 86 minutes. It was shot in roughly 10 minute segments then carefully edited to hide the cuts. This was confirmed in an interview by Elizabeth Olsen and the directors.
Due to the unique nature of the film's presentation as a single take, the production crew ran into several technical issues while filming, mainly surrounding lighting issues and mobility around the house. Since filming was carried out in 12-15 minute takes, there were several occurrences where entire sequences had to be thrown out and re-done repeatedly due to lighting problems or missed cues.
Elizabeth Olsen stated during an interview that she suffered from nightmares during filming simply because she would think of frightening things throughout the day.
When it came down to casting for the role of Sarah, Chris Kentis and Laura Lau sought an actress with theatrical training due to the demanding nature of the filming process; Elizabeth Olsen was selected after impressing Kentis and Lau with her audition.
Elizabeth Olsen is in every single scene of the film and the story is told from her point of view, the film never shifts to another character's perspective.