The Do's and Don't's of Agreeable Conversation : Page 181
pledge not to cut in on each other when either relates what both of them know. If my observation is not faulty, this is a matter in which very, very many couples need a reorientation.
The other unpardonable type of interruption is that which introduces an irrelevant or new topic in the middle of another's remarks. This is a double insult. First it cheats the other of the floor, secondly it shows that the new speaker wasn't listening to the other or brushes his topic off as uninteresting. The speaker is saying that he has trouble keeping the leaves off the lawn, when another pipes up with an observation, such as, "Say, have you noticed how car prices are coming down?" A person who often does that sort of thing is not ready for polite company. If you have ever done this, I trust it was the last time.