I don't recall if I've ever read the DW discussion of fronts. In AW, Vincent Baker is very clear that preparing fronts is
preparing for play, not play in itself, and that it's purpose is
to give the GM something interesting to say. From the AW rulebook (p 136):
A front has some apparently mechanical components, but it’s fundamentally conceptual, not mechanical. The purpose of your prep is to give you interesting things to say.
Baker goes on:
As MC you’re going to be playing your fronts, playing your threats, but that doesn’t mean anything mechanical. It means saying what they do. It means offering opportunities to the players to have their characters do interesting things, and it means responding in interesting ways to what the players have their characters do.
He also says the following (pp 109, 136):
ALWAYS SAY
• What the principles demand (as follow).
• What the rules demand.
• What your prep demands.
• What honesty demands. . . .
Creating a front means making decisions about backstory and about NPC motivations. Real decisions, binding ones, that call for creativity, attention and care. You do it outside of play, between sessions, so that you have the time and space to think.
In AW, preparation of fronts doesn't
change how any move is resolved (though it may introduce a custom move, which typically will be in lieu of what would otherwise be a GM soft move, or perhaps the more generic Acting Under Fire). It certainly doesn't
dictate how any move is resolved. It does bind the GM, by reference to prep, as to what interesting things ("badness", "spots" and "opportunities", in the AW parlance) the GM introduces into the fiction.
Now BW doesn't discuss this issue with the same degree of clarity and precision, but as I replied to
@Faolyn upthread nothing stops a BW GM making notes about ideas for consequences (similar to how Torchbearer
does expressly encourage the GM to prepare notes on possible twists).
I wouldn't be surprised if my GM had already made a note of some sort that prompted the decision, when I failed my roll for Scavenging, to describe Thurgon discovering the letters from Xanthippe to Evard.