"If you are ever in xxx then do look us up," might sound a sincere exhortation to carry the fleeting friendship forward but in reality it is rarely more than a polite au revoir rather than a more final goodbye.
Steven Moffat's new play The Unfriend at the Minerva Theatre, Chichester, seizes on this premise as Peter (Reece Shearsmith) and his wife Debbie (Amanda Abbington) depart their cruise giving their e-mail address to an engaging American fellow traveller Elsa (Frances Barber).
But what do they actually know of their trans-Atlantic voyager over and above a few snatches of conversation on the ship's sun loungers?
When she turns up at their neat estate home in Chiswick to stay with both them and their teenage offspring Alex (Gabriel Howell) and Rosie (Maddie Holliday) they suddenly realise a little too late – thanks to a little Google searching - the potential risk they all face.
What could be heavy drama though is whisked into a souffle of some of the funniest comedy ever served at Chichester.
It may owe more to sitcom than Shakespeare, but this is laugh out loud gold.
The intimate construct of the Minerva might be more appropriate than the Chichester Festival Theatre main house - but it seems churlish to confine something this good to such a small auditorium.
The staging, acting, and direction is as near perfect as it gets - manically pacy and uproariously funny.
Frances Barber delivers a spell-binding performance.
But there is not a weak member of the cast.
While the portrayal of the kids might be a little stereotypical, many of us can identify with their performance and they carry it off with conviction and panache.
Reece Shearsmith, of course, is the master of blending the light and dark as TV’s black comedy Inside No. 9 proves to perfection.
If you are lucky enough to still be able to get a ticket and you are desperate for some good old fashioned fun, this is a must see this summer.