As I'm now hip deep into the process of the third Thorn book, I've developed a lot more respect for writers whose subplots "get away from them", as it were. Because it turns out it's a lot easier for that to happen than I thought. It's now completely clear to me how, for example, Rowling went from the lean-and-tight Prisoner of Azkhaban to the 800-page beast that was Order of the Phoenix.
Both The Alchemy of Chaos and An Import of Intrigue have bits where minor characters from their respective first books have expanded roles. Part of this was out of necessity of the plot-- I needed POV characters for a situation where those characters were the best choice. Part of that comes from enriching the world altogether-- bigger things are happening with the gangs in Aventil, for example, so that means more people and new situations.
So, the big question: how do I keep that natural growth from getting out of control? Because as I reworked the outline for Thorn III, it became clear there were things I needed to at least acknowledge that I hadn't planned on before. Characters who barely existed on the fringes of Thorn now needed a subplot. How did that happen? How do I keep that from taking things over and making what should be a lean action-y novel into a doorstopper?
For me, the main thing is pacing: each scene has to have a definitive this-drives-this-part-of-the-plot purpose. If it's not buying me anything, I drop it. Case in point, in Import, I have a scene where Satrine visits with Sister Alana. Mostly this is to put a pin in there that Sister Alana still exists and that Satrine has rekindled a friendship with her, because she doesn't really have a role in the plot itself. But the scene gives character work on Satrine, her frame of mind in the current case, and advances the plotlines of her personal life. Now, a minor character from Murder who gets bumped up in Import is Corrie, Minox's sister who works the night shift in the Constabulary. I needed her as a POV character because Things Happen where neither Minox or Satrine are present, and hers was the best fit. But that meant I couldn't just POV her when I needed her and ignore her at the later. She needed to stay involved throughout. But, again: it must advance the plot. I'm not going to check in with characters like Reverend Pemmick or Lieutenant Benvin or Commissioner Enbrain just for the sake of doing it.
Now, do I succeed with this? I think most of the time I do. But that's for you all to judge next year when The Alchemy of Chaos and An Import of Intrigue come out.
In the mean time, I've got plenty to do in the word-mines, including finalizing aspects of both those manuscripts, so off I go. See you down there.