Sophie and the Serial Killers' musical numbers are parodies of common metal tropes. The goal of the project was to combine metal-sounding lyrics with typical sounding musical numbers that you would find in Disney movies and such. Common metal lyric tropes are murder, worshiping Satan, being hateful, and being political (typically Left-wing).
About half of the movie was re shot with new co-leads after the previous lead actress had to drop out for personal reasons. Thankfully, most of the cast and crew stayed on board to help re-shoot their scenes and see the project through.
Human Breath's character was inspired by the song "Eaten" by Bloodbath, which is about Armin Meiwes, a Cannibal from Germany. Armin Meiwes put an add up on a website asking for a "well-build 19-30 year old looking to be slaughtered and consumed". One man responded to this, and the two met on December 25th.
Originally, Rikki was supposed to be Sophie's mom. But since the two actress were of different race, it was decided that they would be roommates instead. Some of Rikki's dialogue was inspired by one of the director's old roommates from college.
The screenplay was inspired by battle royale anime's such as "Magical Girl Raising Project", "Future Diary", "Juuni Taisen: Zodiac War", and "Danganronpa".