I remember playing with him one time at Cambridge and he started delivering his usual two-liners, and the audience actually joined in with the punchlines. For example:

DD: Here's a song called 'I'll never get over a girl like you...'
Audience: YOU'LL HAVE TO ANSWER THE PHONE YOURSELF

DD: (nursing a glass of whisky: Just a drop to steady myself...
Audience: SOMETIMES I GET SO STEADY...I CAN'T MOVE

And so on. It was really bizarre. I've often heard audiences join in with songs, but never before (or since) with gags.

He's an incredibly funny man, as well as a terrific guitarist. I wish I had a recording of one of those gigs from so long ago.