>I just designed a new character. She was meant to do nothing more
>than show up and direct someone else to a meeting, and before they got
>there she had a detailed description and the beginnings of an at least
>partially fledged backstory.

This doesn't just happen to writers. While recording the voice work for the Detective Conan (aka Case Closed) anime, the VA for Inspector Megure went "You there, what's your name?" and fellow VA Wataru Takagi adlibbed "It's Takagi, sir!". Officer Wataru Takagi ended up making his way back to the ongoing manga series, and getting several character arcs including a successful romance.