I'm pretty sure he must have a stock of quick responses to certain situations - much of it is in the timing rather than in what he says. He's fantastic on Just a Minute as well, and I don't see how his interjections, with spontaneous audience response, could be convincingly edited in that scenario.
Exactly. Merton has spent the last 20 odd years in situations where he's expected to provide a humorous response on demand. Comedians end up with finely tuned comic "muscles" and can provide a spontaneous quip (or use a filed-to-memory quip) on demand.
People all know someone who is naturally funny but I really expect that, if they sat on any of the popular panel shows, next to Paul Merton, Lee Mack, David Mitchell, Frank Skinner...and so on (any of the other 10 that seem to be on every such show), they'd not get a word in edgeways.
Of all such people I think Frank Skinner is out on his own. I call it Comedy "Tourettes". He seems to blurt a witty reply before the other person has finished speaking. Lightning fast (but he could just have fast recall from a large stored repertoire). Personally I just think Skinner has "funny bones".
So, was Bob Monkhouse witty or did he just have a good memory of his huge joke book? To me he always seemed to be recounting a joke.