Fan Casting Connected All Along in Sort by Tropes

Most works, especially the more populous ones, feature many recurring characters who interact with the protagonists under different circumstances, and who thus wouldn't be expected to know each other. Or, if they do know each other, you wouldn't expect them to have a history together apart from what you see.

Sometimes, though, these characters turn out to have been Connected All Along: they actually have met or crossed paths at some point, or share some other sort of unexpected connection. There are times where it would make sense, they might run in the same circles, even if the topic never came up around the protagonists.