Brad Dourif stole the movie

Yes he did. He was fantastic in this. Interestingly, he pulled in shades of his Exorcist III performance into the character he played in the "Beyond the Sea" The X-Files episode a few years later.
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Never believe. Always question. Rebuke belief, a.k.a. bias, a.k.a. groupthink, a.k.a. ideology, the bane of skeptical, logical reason.

He was fantastic, but don’t forget Ed Flanders and George C. Scott. When I think of both the movie and the book my mind always goes back to the carp scene or the hospital conversation between them. The two of them were so good that now whenever I read the book Flanders=Father Dyer and Scott=Kinderman.

Dourif gave us some of the more intense/ disturbing dialogue and it was fantastic. That being said I think Scott’s ‘I believe’ speech was the most moving and really the crux of the story. They all did a wonderfull job.

Gollum vibes.

He really reminded me of young Jim Carrey.

Maybe so. He certainly stole every scene he appeared in.

I've seen the "Director's Cut" version, as the film was intended originally, and one of the things it critically lacks is Brad Dourif. It's very good thing that Blatty didn't just walk away and instead shot the material required to modify the film. The result is really, really good, even if Father Morning does seem to be visiting from some other film.