I'm not afraid of impromptu conversations and situations, but that is largely because, like an experienced chess player I've been over the usual openings and their variations many, many times.
As Pluto said, stored assorted anecdotes, jokes and illustrations are a definite resource.
(but should not be dumped into a conversation inappropriately, or just to avoid a silence.)
There's always the asking for time ploy. That can sometimes get a very positive (and surprised!) reaction. "I must think about that..." "...give that some thought."
Faced with a topic I cannot (politely) disengage from which is of utterly no interest, rather than knaw my own leg off, I try to find some detail (just a small point or something of a tangent if nrecessary) in order to be able to make a positive contribution. The error here would be to try and make the whole conversation turn that way.