Exactly. The demon was stringing Karras along. Karras is skeptical so the demon enjoyed playing with him. He would give him evidence but would then mentally pull the rug out from underneath him. What the demon really wanted was a face off with Merrin and I think was just having fun with Karras until Merrin arrived.
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And welcome to new and old fans of the movie. Stick around and I think you will be shocked at just how deep this movie actually is.
At times during the film, it can be confusing about who is inside of Regan due to the various voices used. For instance, differing singular or multiple vocal entities emanate from Regan at various times. These are merely a part of the intentional mental illusions that Pazuzu exploits onto his victims in order to reach his ultimate goal of the grand showdown with Father Merrin in the film’s final exorcism scenes.