Regan’s brother – It’s revealed in the book that even before Regan’s (Linda Blair) ordeal, Chris McNeil (Ellen Burstyn) has a strong disdain for the medical profession. This was due to her having lost a son who had been ill and died from an experimental treatment urged by his physicians.
The Medium – In addition to members of the clergy and the Washington elite, a Medium was also present at Chris’s dinner party. The woman immediately senses something is wrong with Regan and tells Chris to get rid of the Ouija board as soon as possible. Chris agrees but says she won’t do it right away since Regan was already dealing with her parent’s separation and didn’t want to take away her imaginary friend, Captain Howdy. The woman later brings Chris a book on the occult: something that eventually finds its way into Regan’s bedroom. This delays Father Karas’ belief in her supernatural possession as he surmises the book led to a psychological power of suggestion.
Regan’s father – Like the film, Howard McNeil never makes an appearance in the book other than to tell Chris he did get a chance to call Regan and wish her a “Happy Birthday.” He alarmingly tells Chris that Regan (already under the demon’s influence) responded to his good wishes with obscenities. It is also hinted that some of Chris’ marriage issues stem from his jealousy of the tabloids’ interest in covering her and her daughter and his feeling left out. What a jerk!
Captain Howdy – As mentioned, Regan’s imaginary friend comes up more often in the book than in the film. It is even suggested by one of Regan’s physicians that she created it to cope with her father’s absence; hence calling it “Captain Howdy” after her father, Howard.
Father Karas relieved of counseling duties – When Damien Karas (Jason Miller) refuses to divulge the names of any troubled priests he counsels who may be responsible for church desecrations, he ends getting replaced. The Church is eager to solve these crimes since they believe a priest is responsible. This is due to clues left at the scenes indicating a more than casual knowledge of Latin as well as Catholic Mass.
Father Lucas – As mentioned, the 2010 updated version of the book features a new character – Father Lucas. This priest is said to have lost his life in an exorcism that took place decades before and foreshadows the demise of Father Karas. Later, he will appear in one of Karas’ dreams and warn the young priest not to perform an Exorcism – presumably a trick by the demon itself.
Twisted Heads – During a conversation with the Detective, Father Karas reveals that in Demonic lore – reversing a victim’s neck was the standard practice of demons towards unfaithful witches. It was believed that by twisting their necks completely around, they’d be unable to give away any of the demon’s secrets.
Detective Kinderman – When the detective first questions Chris (he visits her multiple times in the book as opposed to the one time he does so in the movie) he initially suspects her house caretaker, Karl Engstrom (Rudolph Schundler), as Burke Denning’s (Jack MacGowran) killer. This is due to Engstrom giving a false alibi regarding the night of Denning’s “accident.” Engstrom tells the detective he was at a movie but the film-savvy Kinderman knows the film he claims to have seen wasn’t playing. Later, he’s dropped as a suspect when Kinderman follows him and discovers he has a drug-addicted daughter he enables by bringing money to. His wife, the cook, doesn’t know the daughter is still alive and, given the circumstances, he’d rather she didn’t. This weird subplot would come up again when a fully-possessed Regan would taunt Carl about her.
Voice Recordings – The man reviewing the voice recordings of Regan takes more time analyzing them than he does in the movie. Karas asks that he determine if all the voices could come from a single person and it’s his expert opinion that will eventually convince him to go through with the Exorcism.
Father Karas wins – The book reveals that the movie has a happier ending than many people realized. After Karas falls down the steps and is being given his Last Rites by Father Joseph Dyer (played by real-life priest, William O’Malley) he observes a look of peace and contentment. This suggests to the reader that Karas’ ongoing crisis of faith has at last been resolved and though it cost his life, he was victorious over the demon.