The Amityville horror was found to be a hoax. Investigators found numerous inconsistencies with the story. For example, the Lutz's (the family that was supposedly haunted) claimed that hoofprints were made in snow. Investigators found that there wasn't any snow in the area when these footprints were supposedly discovered.
The Lutz's claimed that there was extensive damage to doors and windows. Upon inspection it was found that there was no disturbance to paint and varnish.
Here's a paragraph which explains what really happened.
"The truth behind The Amityville Horror was finally revealed when Butch DeFeo's lawyer, William Weber, admitted that he, along with the Lutzes, "created this horror story over many bottles of wine." The house was never really haunted; the horrific experiences they had claimed were simply made up. While the Lutzes profited handsomely from their story, Weber had planned to use the haunting to gain a new trial for his client, Ronald Defeo. The Lutzes also later admitted that virtually everything they had said about the haunting-and everything in The Amityville Horror-was pure fiction."
Regarding the murders that did happen:
"For his part, Osuna has his own story to tell. He buys Ronald “Butch” DeFeo’s (a person with psychological and drug problems) current story about the murders, assuring his readers that it “is true and has never been made public” (18, 22). DeFeo now alleges that his sister Dawn urged him to kill the entire family and that she and two of Butch’s friends had participated in the crimes.
In fact, Butch maintains that Dawn began the carnage by shooting their domineering father with the .35-caliber Marlin rifle. Butch then shot his mother, whom he felt would have turned him in for the crime, but claims he never intended to kill his siblings. He left the house to look for one of his friends who had left the scene and, when he returned to find that Dawn had murdered her sister and other two brothers, he was enraged. He fought with her for the gun and sent her flying into a bedpost where she was knocked out. He then shot her."
Regarding the true story behind the Exorcist, I've read several articles plus a book about the matter. Even though the book was written by an author who believed in demons etc., there is much within the book which shows that the true story is nothing like the movie and very possibly had nothing to do with demonic possession. I don't remember details such as names etc., but I'll do by best.
Robbie, the person the book and movie is supposedly based upon, was a thirteen year old boy who used to be very close to his aunt. She taught him how use ouija boards etc. After she passed away he made numerous attempts to contact her with an ouija board. Eventually he started to experience poltergeist like activity. A spiritualist came to the house and said it was his aunt. I don't know if she made an accurate reading.
Noises etc were experienced for about ten days before Robbie started to get effected (one would think that a powerful demon wouldn't need so much time). He didn't get possessed around the clock like Linda Blair's character. In fact, in the daytime he did things such as go to school (not throughout the whole affair) and he had discussions about Catholicism with the priest he worked with. One would think that a person who is possessed by powerful demon wouldn't be able to do such things. There were a couple of occasions where Robbie said its over, only to report differently during the night. Is possession a 24/7 thing or a now or then thing?
The main priest that was involved with the case said that he believed demonic possession was involved because he was required by his faith to make this conclusion (so much for intuitive insight).
Regarding the markings that appeared on his body, there are mixed reports. Some say that evil markings mysteriously appeared on his body. One report said that a man caught Robbie make a marking. Some reports say that people saw nothing but rashes. At one point "no school today" appeared on Robbie's body. Think about this one.
Even if such markings appeared on Robbie's body without him making them externally, it is possible that his subconscious mind caused them to appear. Hypnotists have found that they can cause people to do such things. I wouldn't be surprised if Robbie was just simply visited by a spirit that used to be human, he (Robbie) heard about demons and such from the priest who worked with him, took on a belief system accordingly, and then his subconscious mind caused the markings to appear. Even if a spirit visisted him and caused the markings to occur, there is no evidence whatsoever that something such as a demon was involved. Just as physical satanists can write and say heretical things, so could a confused spirit who used to be human.
One other point. Whatever it is that caused Robbie to go throught the difficulty he went through, none of the people that were around him and tried to help him were troubled by an evil spirit. One would think that if something such as armies of darkness exist, the demon that supposedly troubled Robbie would've called for reinforcements.
I'm not saying that hauntings never occur. But I believe it is a mistake to believe what Hollywood has to say about them, and to assume something such as demons are involved.
P.S. Is this

a demon or a smiley face with an attitude problem?