Ok - I'll be a bit Victor Meldrew and then I'll shut up, I promise.
My PA's are nicely polished by now - the occasional laugh (more of a smile really) but low key and not frequent enough to irritate.
The one time I don't due humour is during the Safety Briefing: I train others to the same, and tell them off if they try. I just don't believe that the text of the Safety Brief is the right place - ever! Beforehand, by all means, set it up, get their attention: Afterwards, fine - but during, never.
OK - now I've rained on your parade (please accept my apologies) I'll shut up and let others get on with it - and there are many more gags, believe me.
P.S. - Who'll be the first to do the Paul Simon ways to leave your lover?