I have felt this "like talking to a movie and wondering why the characters aren’t responding." feeling but I did not know how to put it into words. There are people who cannot and will not understand what you are saying. You put it best.

Sometimes I say things to those kinds of people, even something that makes no sense to see if I can kick their brain into think mode even for a second. They usually agree and move the conversation along, others glitch out for a moment and then pretend they had not heard anything.