 In the initial stages of the film, Jason Miller (I) (qv) was not  cast. In fact, scenes of Father Karras death were shot using  Brad Dourif (qv). One scene in particular had  George C. Scott (qv) examining Dourif (Karras) body on an  operating table after the priests death. No movement, however there  is a sound from an EKG in the room. The sound suggested some sort of  brain activity... Jason Miller was cast because the producers  decided that, after showing the film to test audiences, a member of  the original cast of _Exorcist, The (1973)_ (qv) should appear in  the film. When this decision was made, William Peter Blatty (qv)  rewrote parts of the script, particularly those involving the Gemini  Killer. The result is the schizophrenic conflict between the Gemini  Killer and Father Karras (Dourif and Miller respectively) that  exists in the film.