Okay, The Exorcist is a good movie but it gets some things wrong. Like, I mean ALL wrong.
Real confrontation with demons isn't some ritual you follow from a book. Ceremonial prayer doesn't really do anything. Neither can you just throw "holy water" at it and think it's gonna burn the thing like battery acid. In fact you will probably make it laugh at you for trying that.
You never tell it your name. You don't even ask IT for its name. Yes, it has one but you are not interested in what its name is, period. You don't answer anything it asks of you. You close your ears to it no matter what it tells you. You DO NOT dwell upon any kind of knowledge or prophecy it screams at you. I can't emphasize that point nearly enough. It can tell you how you're going to die but you have to deafen your ears and mind to that. Else you obsess with that and then you’re no good to anyone who’s involved in this with you.
Now, some things the movies do get right, like when it's talking in some other language, but don't expect to be able to translate it. I'm just saying it can do that is all. But do not expect for it to make someone's head turn a full 360 degrees like Linda Blair's did. The inhabited person will sometimes display feats of strength not in proportion to sex or physical size, though.
A drop in ambient temperature has been reported in some cases, but in general this doesn't seem to happen very often.
Oh yeah and it does sometimes produce vomiting, so be ready for that.
There aren't too many people really strong enough to handle this sort of thing. If you aren't that strong, be somewhere else, okay?
I wonder if anyone in the film industry will ever make a movie about what fighting a demon is really like.