Mercedes McCambridge recalls her days on shows like "I Love a Mystery" as well as the secret of how she found the voice for the demon, Pazuzu, in the original version of THE EXORCIST.

The woman was Carmilla's "aunt" (or, at least, one of the accursed Karnsteins.  

Another thing that should be mentioned, is that while Mercedes' voice may have lost some of its flexibility by the 1960s and 70s her talent as a radio actress remained. You would know how difficult it is to become a character that lives in the imaginations of others by just using your voice if you had ever attended one of her workshops.

And one more thing, Mike B.  the intention of the actors is not to "fool you" they only give you a chance to set your imagination free. That you refused to take advantage of that is forever your loss---not theirs.

And, by the way, she isn't playing Carmilla.
