Spoiler Alert: This article contains spoilers for Osgood Perkins’ “Longlegs,” which hits theaters July 12.

Although Alicia Witt is a key player in the mystery of Osgood Perkins’ satanic horror film “Longlegs,” she made sure to steer clear of the screening at Monday night’s premiere.

Witt described her time shooting “Longlegs” as “the most cathartic experience” of her life, during which she fully surrendered to her character Ruth Harker, the maniacal mother of Detective Lee Harker (Maika Monroe). Having had to facilitate several unholy killings throughout the film, Witt tapped into the “darkest material” she had ever accessed as an actor to prepare for the role.

“When I did get the role, I had about six weeks. I journaled, I made playlists, I thought about her thought process, what she experienced as a child and what her darkness was,” Witt explained. “By the time I got to set, I could...