Answer: the listener should jot down notes about what the speaker is saying.
In a debate, one of the worst thing the listener can do while the speaker speaks is to try to come up with counterclaims to win the debate. While this seems counter-intuitive, focusing on giving an answer often means the listener is not paying attention to the speaker, and analyzing his points properly. It is also unfair to try to distract the speaker by moving around or being disruptive. The correct thing to do is to pay attention and take notes. This way, you can really listen to the speaker and understand what he is saying.