The telling of any character is what they do in a different situation.

Occasionally a particular word or phrase in a letter or diary has sparked an entire plot - like an echo from history, still very alive.

Archive material is a fabulous starting point - individual documents are like signposted roads, heading to a variety of intriguing possibilities.

If you put Mirabelle into some of the situations she gets into, there is only one way Mirabelle can behave.

I have no problem in moving a date one way or another or coming up with a subplot that gets my characters in (or out) of a fix more rambunctiously than the extant records show.