The cause in the film seems to be because Regan found a Ouija board and played with it.
from wikpedia "Selma Blair Beitner in Southfield, Michigan,[1] the daughter of Molly Ann, a judge,[2] and Elliot Beitne"
you maybe be controlled by the devil but don't hail the devil you will be hailing the wrong demon :(
The most famous is The Excorsist And a bunch of B movies.
Her name is Regan MacNeil.
For her movie The Excorcist. She was VERY good in that And she was just a teen!