One just pulled together on the realization that one character is the main one, and therefore it would really benefit from simplifying the back story of another, even though the central plot is that the main character gets entangled in the events that started in the other character's life.
Which simplifies down to a robbery, which the other character witnessed.
And as soon as I made it a philosophic engine that was stolen, and decided, pulling something out of a hat, that it would control fire, that the story obediently informed me that the reason it was crucial was that such superior fire could be used to forge more philosophic engines. Which is dangerous. Such engines can threaten the structure of the world. And provides a good motive for the rest of the plot.
'cause sometimes the inspiration just flows.