The 1973 movie The Exorcist starred Linda Blair as a child named Regan who becomes possessed by a demon. The film was adapted from the horror novel by William Peter Blatty, and banned in the UK for more than fifteen years. It is referenced a few times in Buffy and Angel:

• In Teacher’s Pet, Buffy says that Miss French’s head did a “full-on Exorcist twist”, referring to a famous scene in the movie.
• In I Only Have Eyes For You, When Willow suggests an exorcism, Cordelia replies “Are you crazy? I saw that movie: even the priests died!”
• In Lover’s Walk, Buffy says of her mother, “her head spun round and exploded” (to which Giles replies, “I’ve been on the Hellmouth too long. That was metaphorical, yes?”).
• In Living Conditions, Xander says, “Are you saying that Buffy’s been doing a Linda Blair on us because Kathy’s been sucking her soul?” At the beginning of her possession in The Exorcist, Regan showed dramatic changes in her behaviour, just as Buffy did in Living Conditions.
• Buffy name-checks The Excorcist in Bring on the Night.
• In the Angel episode I’ve Got You Under My Skin, Cordelia says, “I’m wondering if I should put plastic down. Angel, are you expecting any big vomiting here? Because I saw the movie.” Large parts of this episode are a nod to The Exorcist - in the book and movie, a Catholic priest dies in much the same way as Angel’s Father Fredricks.