Here's Linda Blair's actual performance as recorded on set, followed by the same scenes dubbed by the great Mercedes McCambridge in the released film.

From the Associated Press:

McCambridge was hired to portray The Demon in William Friedkin's 1973 smash hit The Exorcist. After weeks of what she called the hardest work she had done for a film, she had been promised prominent mention in the credits. But when she attended the preview, her name was missing. As she left the theater in tears, Friedkin tried to explain that there had been no time to insert her credit. The Screen Actors Guild intervened and forced her inclusion in the credits.