Voicing the demon

Dubbed "the world's greatest living radio actress" by Orson Welles, Mercedes McCambridge voiced the demon Pazuzu that possessed Regan in the movie.

In order to achieve the sinister and dark timbre, McCambridge chain smoked, drank whiskey non-stop and only ate raw eggs to get a suitably raspy voice for the evil entity.

She later explained: "You utilize everything, don't analyse, utilize. And I utilized the thickness all of that stuff [for] the voice of Lucifer."

As if this wasn't enough, the actress also insisted she was to be tied to a chair with pieces of a ripped sheet around her arms, legs neck and feet so she could get into the mind of the possessed Regan.

Friedkin recalled: "The most curious things would happen in her throat. Double and triple sounds would emerge at once – wheezing sounds, very much akin to what you could imagine a person inhabited by various demons would sound like."