According to the film's production notes, Bertino was heavily inspired by the true crime book Helter Skelter by Curt Gentry and Vincent Bugliosi, which detailed the Manson family murders.
"I was thinking about the Tate murders and realizing that these detailed descriptions had painted a story of what it was like in the house with the victims. But none of the victims knew about the Manson family or why it was happening to them. So, I got really fascinated with telling the victims' tale. And not filling it in with an FBI profile and not filling it in with finding out that somebody's grandmother beat them and now they want to kill everybody. You read obituaries every day where someone is killed for a random reason. Yes, we may eventually find out why, but sometimes they don't."
Per Listverse, Bertino devised the film based on a childhood memory. The memory in question: young Bryan was sitting out on a street near his neighbor's home when someone knocked on their door. "As a kid, I lived in a house on a street in the middle of nowhere. One night, while our parents were out, somebody knocked on the front door and my little sister answered it. At the door were some people asking for somebody who didn't live there."
It was later realized that the unknown individuals were going to houses and breaking in, although events tied to the incident weren't linked to any murders. Talk about a stretch.
ncG1vNJzZmivp6x7qL7Up56eZpOkunB%2BkmptcG5fqbWmedGemKVlnJ6zpnnEr5ynrF2ptaLAjK2fnmWjqb%2Biusaeqaxlmah6o63Snptmp55k