That's it. I've got it. I might have mentioned that when writing the first draft of Dustfall there was a point around the 20-25k mark where all the character dynamics clicked into place and I realised what the plot was about. In that case I realised that the three main characters represented three different perspectives on a spectrum: one looking at life for the benefit of the individual, one concerned only with the society as a whole, and the lawkeeper figure who's meant to bear both perspectives in mind and determine where to compromise between the two.
I've just had the same sort of epiphany for Picketwire - and coincidentally it's also at a similar mark (21,597, as it happens.) I've been aware that so far my protagonist Mal has been pretty passive with little distinctive identity coming out. I've been finding whenever she speaks to someone the conversations are almost all one-sided in favour of the other person, with her rarely contributing more than encouraging tokens to keep them going.
But now I've seen what this novel is about. I've known for a while that one of the core themes is that this city is on the cusp of an ideological revolution - I usually describe their tech level as 'within a generation they'll have turned steampunk'. Even in my plot outline on the NaNo website I mentioned that the murder mystery that takes place within the story is in one sense incidental to the social changes it kicks off.
Given this setup, and plenty more besides, it's now staggering that I hadn't realised I needed to put the main characters on another spectrum: this time of the status quo. Effectively whether the characters are looking to the past, present or future. And the great thing about it is it makes perfect sense with all sorts of hints I've thrown in so far - for example the Heritagist movement (essentially the same as the Medievalists from the early 19thC - see here) is the past, most of the proletariat are the present, and Mal now becomes a futurist. Which works well, because she's young and student-y and hasn't been comfortable with the adult status quo for long enough to have much emotional attachment to it. (And if the novel ends the way I think it will, it'll have much more of a bittersweet resonance than I'd initially had.)
I wonder if this happens to most authors - that, no matter how much they think about the thematics at the start, halfway through they'll find a way to reinterpret the novel completely and add entirely new thematic levels. That their genre story suddenly becomes more suffused with Meaning and Purpose and all those other good bits that are great to justify why they're spending so much of their free time writing this novel.
"It is as easy to dream up a book as it is difficult to put it on paper."
Balzac (which should be pronounced as it is in The Music Man - distastefully, emphasis on "BALL-sack"

