Tuning out

"You have a way of saying you're going to listen to something, but then when the other person is talking you're up in your head having some sort of complicated policeman dialog with yourself about something else."

The speaker is describing Hank Palace, a former cop who can't stop being a cop, in the second part of The Last Policeman trilogy, Countdown City.  If you swap the word 'policeman' to 'academic' it's a sentiment I've heard dozens, possibly hundreds, of times.  I can happily spend hours upon hours locked away in my head talking to myself!  The book, by the way, is proving to be as entertaining as the first.