From Collider.com:
The story was ground zero for the popularity of possession plotlines. Following the possession of 12-year-old Regan, played by Linda Blair, The Exorcist heavily relied on special effects makeup and practical effects to convey the gravity of the on-screen horror.
However, as revealed in his 2023 Vanity Fairy interview, Friedkin notes how Alain Resnais’ films inspired him to include subliminal imagery to elevate The Exorcist’s tone of terror. Spliced-in clips and sounds that don’t belong in the scene appear throughout the film, sometimes caught by the audience, sometimes not. Regardless of whether a viewer consciously catches these flashes of horror, the subconscious brain does.