We all do this sometimes—it’s a survival tactic and probably largely instinctual: if we move very slowly and pretend nothing is wrong and make no sudden moves maybe we’ll be able to conclude the conversation and get out the door before the insane person we’re talking to really flips out. Some of the stories and novels I love the most do this as well, with the characters either not responding or responding differently than you’d expect. At its best, what that does to you as a reader is make you wonder about your own sanity: if the characters see it as normal, shouldn’t you?
We (by which I guess I mean I, though I hope I’m not alone in this) have all been in relationships where years later we look back and don’t recognize our actions, can’t really understand why we were willing to go along with the other person’s ideas or behavior which, now that time has gone by, we recognize as delusional. Since humans are imitative animals, we often take our cues from the people around us. If you do that long enough with the wrong person, you enter into the sphere of their insanity in a way that makes you regard the insanity as the new normal.