In the novel it is suggested that there may be a psychological factor in her use of that particular name, since her recently divorced father's name is Howard. It's also suggested through the various drawings that Regan makes that the demon Pazuzu has appeared to her in dreams. We learn that the demon inhabiting Regan actually has a name, Pazuzu and an actual cultural history. In Assyrian and Babylonian mythology the god Pazuzu was the king of the demons of the wind, and son of the god Hanbi. Pazuzu is often depicted as a combination of animal and human parts. He has the body of a man, the head of a lion or dog, eagle-like taloned feet, two pairs of wings, a scorpion's tail, and a serpentine penis. He is often depicted with his right hand pointing upwards and his left hand downwards. In "The Exorcist" this is the demon that inhabits Regan. As the story unfolds we find out that Pazuzu had been confronted many years earlier by Father Merrin during an exorcism of a boy in Africa. And now in present day Merrin is called again to confront this demon in young Regan Macneil. Pazuzu is explored in more detail in some the sequels, Exorcist II: The Heretic and the alternate prequels Dominion: Prequel to the Exorcist or Exorcist: The Beginning.
There are several scenes in which the viewer can see the face of a demon flashing quickly on the screen in The Exorcist. The easiest to spot, even with the naked eye, comes during Father Karras's dream sequence, right after the shot of a figure walking up a subway staircase. The demon mask used in the movie Onibaba (1964) inspired William Friedkin to use a similar design for the makeup in the shots. In each of the shots the demon Pazuzu is played by Linda Blair's stunt double, Eileen Deitz. Deitz, who underwent makeup tests for the "possessed Regan", wore one of the alternate make-ups in her scenes as the demon.
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