I am so confused. I feel like this is a complete contradiction. Earlier you said that the PCs should be able to answer any question they want, and the Keeper should produce the evidence to do so.
No. They should be able to
ask any question they want (specifically from among the choices allowed by the move, a limitation that is very important), and the Keeper should answer it honestly. But "honestly" doesn't have to mean "completely." Use the fiction to give them clues. Where the inventing-things-in-the-fiction part comes in is in how you answer those questions.
Let me give you a more concrete example that might better illustrate what I'm talking about. Let's say the monster in question is a murderous ghost. It is generally incorporeal and/or invisible, but it can manifest in order to make its homicidal fury known. The PCs are investigating an old house where said murderous ghost has despatched a member of a work crew doing remodeling - seems the ghost didn't like having that wall knocked out to make the floorplan more "modern, open, and airy." The PCs check out the scene of the crime, looking around for any clues as to what happened here. They talk about how they're carefully casing the joint, looking for any physical evidence. The body is already gone, but they have crime-scene photos and the police report, which they are cross-referencing now that they are actually on-site.
The Keeper decides that this description of the PCs' actions is sufficient to trigger
investigate a mystery and calls for a roll. Since everything they've described so far sounds like standard police work, the Keeper calls for the Expert to make the roll. The player of the Psychic decides that she's using her psychometry skills to try to spot anything the police might have overlooked or that the Expert might otherwise miss. The Keeper decides that this constitutes
help and has the Psychic's player roll that. She succeeds. With the Psychic's help, the Expert gets a 10, which gives him 2 questions. First, he asks,
"what sort of creature is it?"Knowing that it's a murderous ghost, the Keeper needs to come up with some way to communicate that. Being incorporeal, the ghost can pass through walls, so the Keeper describes how the workman was in the house by himself, but in the police report all of the doors were locked; the rest of the work-crew had to break in the following morning prior to discovering the body. Other than the front door they broke, there are no other signs of forced entry. So whatever this thing was, it's the kind of creature that doesn't care about locks or walls. NOTE: the Keeper doesn't simply say, "it was a murderous ghost," because while that is true, it is also not interesting. By saying what he did ("the kind of creature that doesn't care about walls or locks"), the Keeper is answering the player's question honestly and providing them with information, but not necessarily taking all the mystery out of it. At least not yet.
Deciding to throw the Keeper a curve ball, the PC's now ask their second question:
"what is being concealed here?" The Keeper knows the ghost is angry about the changes to its environment, and decides to communicate a hint of this to the players. So maybe the Keeper decides to get the helping character involved and says something like: "As Carla the Psychic is running her hand along the wall the workman was demolishing, she gets an overwhelming impression of joyous, childlike laughter. When Gunner the Expert examines the wall, he sees that it's many layers of paint over a couple layers of wallpaper over the original lath-and-plaster. On one particular fragment of the wall lying in a pile of debris, those layers are hanging loose. There, under all those years of coverings and drawn on the original plaster are fragments of little doodles. Ripping off paint-covered paper from a part of the wall still standing reveals more, including a little heart drawn around the words 'Cassie loves Daddy.'"
Man. No wonder the ghost was pissed. But at this point, the PCs have some clues as to what the creature might be and some insight into the motive behind the murder of the workman, but they don't know for sure that it's a ghost. Could be something else.
Or we could go a different way. Say instead the players decide that they want to cut to the chase and try to find this monster as quickly as possible. Right, so it can get in (and presumably out again) with ease. That makes tracking it hard, maybe. So maybe instead for their second question they ask,
"where did it go?" The Keeper knows that the murderous ghost is tied to the house, and it hasn't gone anywhere. It's right here. Right now. Maybe paying attention to them, maybe just chilling on the ethereal plane endlessly reliving its last days in a fugue state. But the Keeper looks at the situation, again sees that the Psychic is helping out, and just goes for it, narrating: "As you move around the house, you encounter cold spots. Or rather, spots that are sometimes cold and sometimes not. Weird. But then, while touching the old mantle over the fireplace and using her psychometric abilities, Carla hears clearly as a bell a low, gravelly voice saying, 'Get. Out.'"
The PCs can look around the room and see that there's no one else here! Oh, shit! The monster is definitely paying attention to them now! And in answering the second question honestly, the Keeper has actually reinforced the answer to the first. Further, this situation has just snowballed in a way that it might not have had they asked a different question (e.g.
"what is being concealed here?"). So now they might know (or strongly suspect) that it's a ghost, but by not asking the other questions, they have no insight into its motives. They might also have a fight on their hands, one for which they are probably ill-prepared.
Now you are saying it should be limited. If we're just poofing up whatever evidence we want, then surely then can run some test to figure out exactly what monster it was, or the victim just happened to drop their diary describing exactly the monster they were hunting, or something like that.
Not at all, and I hope the above example clarifies that. You are giving the players clues, not revealing the entire mystery. Which questions they ask help you decide which clues to give them and how. You can be as vague or specific as you like given the fictional situation, but you always have to be
honest. If the creature is a ghost, don't answer
"what sort of creature is it?" by describing puncture wounds in the neck and a serious lack of blood. Talk about ectoplasm, or cold spots, or weird electromagnetic effects, or the presence of limestone. If you want to be vague, talk about eyewitness accounts of someone seeing a shadowy figure standing at the foot of their bed. Ghost? Demon? Or maybe some creature that manifests not in the real world but in dreams? Leave as much uncertainty as you like, but don't lie.
And, again, you seem to be saying that all the PCs should declare all their actions up-front before starting resolving things. That really isn't how I'd pictured it working it all. I'd figure "Help Out" would go something like:
Beth's player: "I'm going to dig through the files and look for anything unusual about the murders".
Jasper's player: "OK, I know the organization of this place well, so I'll help with cross-referencing details and stuff".
GM: "OK, sounds like Jasper is "helping out", so roll to see if you help Beth.
Whereas two characters investigating would look like:
Beth's player: "I'm going to dig through the files and look for anything unusual about the murders".
Jasper's player doesn't interrupt.
GM: "OK, Beth, roll to Investigate a mystery" (rolls, asks some questions)
Jasper: "While she's doing that, I'm going to go down to the lab and run analysis on those scrapings I got earlier".
GM: "OK, Jasper roll to Investigate a Mystery" (rolls, asks some questions)
So do you go with declaring all actions first?
Typically I would, yes. If one of the PCs is doing something that might be involved or difficult or time-consuming, it's totally fair game to say, "Hang on a sec. Before you roll, what are the
rest of you doing while Gunner starts walking the scene and comparing it to the forensic photos?" Get their input and decide what sounds like helping and what sounds like an entirely different "investigation."
And what would keep them from just investigating some more (assuming there isn't an immediate pressing time limit; ie, the creature only comes out and night and they are investigating first thing in the morning)?
Because the fictional situation hasn't changed.
IMPORTANT: Here's something you need to understand about PbtA games: "I do it again" is almost always the wrong thing to say. Once you've made the move to
investigate a mystery, you've investigated it. You will note that there's no move to "re-investigate a mystery," so there is literally no mechanism in the rules to "just roll again and ask more questions." You've found what you're going to find, and no amount of poring over those files or photos or transcripts or physical evidence is going to turn up fresh insight.
You should also note that there is no "duration" mechanic at all in MotW. How long does it take to investigate something? As long as it takes. Could be minutes, hours, or days. Mechanically speaking, it doesn't matter. But
fictionally speaking, it does. And once it's done, it's done. You can't just do it again, because the fictional trigger that led to you making the roll in the first place no longer applies.
But if the fictional situation changes - a new body turns up, the players meet someone who gives them new information, new evidence is found, etc - well, that's a
new mystery, and it can be investigated. And this gets to your earlier question about multiple investigative scenes within a story.
Is this making more sense now?
That's why when you say things like...None of the mechanics actually cause unexpected things to happen - its just all based on player input and GM fiat
...I quite literally have no idea what you're talking about. Because it's not just "GM fiat" if you're doing your job well - it's a consequence of the fiction. And because the players have input into that fictional landscape, the results of following that fiction can come as a surprise to everyone involved - including the GM.
GM fiat is the GM making a decision, as opposed to a rule or mechanic doing so.
A pistol doing 2 damage is a rule.
A collapsing bridge doing 10 damage is GM fiat.
Don't get confused between
rules and
mechanics.
Sure, the book says your garden-variety semi-auto pistol does 2 damage. But under what circumstances do you apply it? If your character is struggling over the pistol with some NPC and misses the roll? If your character is struggling over the pistol with some NPC and gets a partial success? If the NPC has made it clear that he'll shoot unless the PC backs up and gets the fuck off his front doorstep and the PC insists on continuing to try to sweet-talk his or her way inside?
By the rules as written, these are all valid applications of the
rules for applying the damage
mechanics. The first is a player miss for a move (with struggling for the gun being a pretty standard example of
doing something under pressure), which presents the "golden opportunity" for a "hard move" (i.e. a Keeper move where the fictional situation and its consequences are narrated at the same time). Not only did you not get the gun, you got shot. Suck. The second is a partial success on
doing something under pressure, for which the Keeper can offer a worse outcome, a hard bargain, or an ugly choice; a hard bargain is something that comes at a cost. Well, you managed to get the gun away from the guy, but not before he shot you with it. Ouch. Now what? And the last is the result of the Keeper setting up the fictional situation. The guy warned you. You didn't get off his porch. So he shoots you. Man, Castle Doctrine and Stand Your Ground laws are a bitch.
In all three cases, the Keeper is making a move -
inflict harm as established - which the rules say he or she can, and lay out guidelines for the circumstances under which it's appropriate. In some sense it's GM fiat, but circumscribed by the GM's Principles and Agenda.
But how is that any different than the GM fiat involved in deciding in the first place whether that NPC had a pistol or a shotgun? How is it any different from the GM fiat involved in placing the players in a situation with an armed, angry NPC?
Constructing a setting entails scads of GM fiat; making that setting realistic (
make the world feel real), engaging (
make the characters' lives not boring), heroic (
be a fan of the characters), and surprising for everyone involved (
sometimes, disclaim decision making and
play to find out) entails putting boundaries on that GM fiat. That's what people are talking about when they say, "follow the fiction."
Does that help?