The Unreveal of whether Pazuzu was impersonating Damien's mother, or channeling her from Hell.
Fr. Merrin, the exorcism expert, believes the appearances of Mary and Burke and the "three personalities" Karras thinks are present are just more of Pazuzu's deceptions and "there is only one" demon.
The movie doesn't even need anything supernatural to be terrifying; witness the scene of Regan getting a lumbar puncture.
Oh God. This troper could handle the demon's scenes easily, but it was the hospital scenes that made her cover her eyes in terror. Even more so since Regan doesn't seem to understand just why they're running so many tests on her and is frightened by it to the point of screaming and fighting the doctors.
In the book, Pazuzu mocks Karras for injecting Regan with so much Librium. He tells him (correctly) that he's going to induce a heart attack in the girl if he continues to.
Regan matter-of-factly telling an astronaut during a party, "You're gonna die up there". (Her urinating right after was creepy, but the way the line was delivered... brrrrrrrr.)
The Exorcist II
Father Philip actually punches his way into Pazuzu!Regan's chest and crushes her heart.
The Exorcist III
The Dream sequence; it has a very eerie, tranquil while sounding off key feel to it at first and then gets frantic and more overtly horrifying right before Kinderman wakes up.
Nurse Keating's death, one of the best Jump Scaresever.
In one scene, a statue transforms somehow with lighting into a Monster Clown with a dagger.
The Gemini Killer knows how to Hannibal Lecture better than the Trope Namer. Listen to how he's able to cut someone's head off without a single drop of blood being spilled. His description is worse than seeing it.
The part where he randomly starts singing in a high falsetto voice, and then abruptly stops. Both him and Kinderman act like nothing happened.
The exorcism scene; it may only be there due to Executive Meddling, but at least the crew had the decency to make it downright unsettling.
particularly horrifying is when a statue of Jesus being crucified is desecrated to mock the murder of Thomas, a young boy.
That wasn't a part of the added scene. You'll notice Father Morning is nowhere to be seen.