The novel also describes Karras walking into Regan's bedroom for the first time, the stench in the room was so awful, it was like a blast of foul odor that hit him as he walked through the door. Karras' eyes watered, and he had to hold his breath, the smell was so bad.
There was no indication of this in the movie, though. Karras just walked in. The part in the book where possessed Regan takes a dump in her diapers wasn't in the movie either, but that's probably for the best. It may have been unintentionally funny, seeing Regan screwing up her face and you hear a muffled “fffuh” defecating sound!
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(Oh dear lord, I can't believe what I'm typing here! ^^)