You also have to hand it to Franken - I've been in a few of these sorts of situations myself, and the very first order of business is preparation. He knows the arguments, if not all the technicalities, cold. You have to have examples and contrary positions already worked out and waiting to be called to the front, as he shows. He has to have prepared himself to concentrate on the conversation and not ever loose focus, regardless of the left field comments, but he still has to be listening. Part of this is, of course, standard training for any on-stage show business type, as well for any politician. Here they compliment each other.
Also, it's one thing to shout at a politicain at a podium 30 feet away. It's another to do it when he's standing right next to you, with friends and neighbirs standing around. The crowd dynamics are strikingly different.