Humorous aside: I just asked Miss M a grammar question. We discussed possible correct answers, parts of speech, etc. She asked, “Where is that book?”
I just looked at her. “That book,” I said. Even allowing that the possible world had been narrowed down to grammar books– a not very narrow space here on the Farm– “That book” was pretty ambiguous, especially since I’m a Harbrace man & she’s Little/Brown and we have multiple editions of each.
Turns out I did not need a comma.