If people in the town have been losing faith because rumours about heretical papers that are claimed to prove that the prophet is a fraud and the faith is based on a lie, it is perfectly reasonable for the PC to confront the mayor who is clearly part of this heresy and have a goal of "I compel him to divulge the truth about the location of heretical papers."
Except that the prep might say that it is only the undertaker that knows the location of the papers, thus the mayor does not possess the truth to divulge.
And that is limiting the action declaration based on secret myth!
To begin with a general statement, I hope it's obvious that different RPGs give different answers to the general question,
how is fictional position established. And that even the same RPG might answer the question differently in different contexts: for instance, in MHRP it is generally the GM who establishes fictional position in Action Scenes, but the players enjoy a lot of leeway to establish fictional position in Transition Scenes.
Today I played in a session of Burning Wheel. No one just declared "I stab the Orc", because at no point did anyone's fictional position include their PC's proximity to an Orc. How, in Burning Wheel, do we establish that fictional position includes proximity to an Orc? Not just by making
an Orc shows up roll: there is no such roll. The GM has to frame it, or a player succeed at an appropriate Circles check.
"I compel so-and-so to divulge the truth about such-and-such" depends upon the fictional position including
that they know the truth or at least
that it is possible that they might know the truth. How is this fictional position established? In DitV there is no "such-and-such NPC has such-and-such knowledge" roll. What NPCs know is under the GM's control.
But in DitV, the GM is
actively revealing the town
in play. Thus, if the undertaker has the secret information, the GM will be revealing that. From p 139:
The PCs arrive in town. I have someone meet them. They ask how things are going. The person says that, well, things are going okay, mostly. The PCs say, “mostly?”
And I’m like “uh oh. They’re going to figure out what’s wrong in the town! Better stonewall. Poker face: on!” And then I’m like “wait a sec. I want them to figure out what’s wrong in the town. In fact, I want to show them what’s wrong! Otherwise they’ll wander around waiting for me to drop them a clue, I’ll have my dumb poker face on, and we’ll be bored stupid the whole evening.”
So instead of having the NPC say “oh no, I meant that things are going just fine, and I shut up now,” I have the NPC launch into his or her tirade. “Things are awful! This person’s sleeping with this other person not with me, they murdered the schoolteacher, blood pours down the meeting house walls every night!”
...Or sometimes, the NPC wants to lie, instead. That’s okay! I have the NPC lie. You’ve watched movies. You always can tell when you’re watching a movie who’s lying and who’s telling the truth. And wouldn’t you know it, most the time the players are looking at me with skeptical looks, and I give them a little sly nod that yep, she’s lying. And they get these great, mean, tooth-showing grins — because when someone lies to them, ho boy does it not work out.
Then the game goes somewhere.
The GM will also have in mind the following principle, from p 138:
If nothing’s at stake, say yes to the players, whatever they’re doing. Just plain go along with them. If they ask for information, give it to them. If they have their characters go somewhere, they’re there. If they want it, it’s theirs.
Sooner or later - sooner, because your town’s pregnant with crisis - they’ll have their characters do something that someone else won’t like. Bang! Something’s at stake. Launch the conflict and roll the dice.
The core of DitV play is not
discovering what is wrong. It's
doing something about it. This is why the rulebook emphasises the need for the GM to withhold judgement about the players' responses, while also pressing them “really? Even now? Even
now? Really?” (p 141). For instance (pp 122, 124),
Your goal in the next town is to take the characters’ judgments and push them a little bit further. Say that in this past town, one of the characters came down clearly on the side of “every sinner deserves another chance.” In the next town, you’ll want to reply with “even this one? Even this sinner?” Or say that another character demonstrated the position that “love is worth breaking the rules for.” You can reply with “is this love worth breaking the rules for too? Is love worth breaking this rule for?”
So if the GM is following the principles of the game, it won't come about that the players
think their fictional position includes that the mayor might have the information, although the GM
knows (from prep) that in fact it does not, because only the undertaker has that information. The GM, with the goal of
driving play towards conflict by
actively revealing the town
in play will have communicated this.
If things have got to the point where the players are confused about fictional position, such that they think it is possible that the mayor has the information even though prep states that only the undertaker has it, then as I said the GM has made an error.
The difference from CoC is obvious, I think. It's basically the opposite: CoC takes it for granted that the players will share the judgement of the source material that the cults and blasphemous truths and so on are horrible, to be destroyed or hidden or avoided; and the principle goal of game play is to learn
exactly what is going on.
Whereas in DitV, the GM is expect to reveal what is going on so that the players can then
form a judgement about it.
The same is true of The Blue Cloak. The point of the scenario is not
that the players, as their PCs, work out that the bandits murdered the merchant. It's what they do about this. What does it mean to be a knight errant in the time of King Arthur?