in Publish or Perish..the scene where Columbo is eating chili in the fancy restaurant questioning Mariette Hartley and Jacques Aubuchon...
when he goes to leave...he starts to get up and sits back down...stating.."Im having a hard time getting up"...both actors seem to snicker a bit....funny subtle moment
I don't know about improvised, but one moment that strikes me as funny is when he's in that hospital waiting room with Milo Janus and he grabs that magazine. It's funny because, what are the odds of someone that infuriated managing to read even one word of the thing? But it also makes sense, because that's just "what you do" in a waiting room. So I wonder whether that was Peter's idea or not.