Most arguments rehearsed in the shower are won. That isn’t surprising, since you’re writing both parts, and the other person, who in real life would say something you hadn’t thought of, says here exactly the thing you have a perfect answer to. So the rehearsal mostly trains you for a conversation that will never happen. The useful version is harder and less fun: give the absent person their best lines, the ones you’d rather they never thought of, and see whether you still have anything to say. Lose a few in there. It’s warm, and nobody’s listening.