Based partly on the storytelling mechanism of 1001 Nights, and partly inspired by Shadows, I've come up with a little narrative, diceless mechanic:

Each player takes it in turn to narrate a scene, but at any point, any of the other players can attempt to steer the action in a particular direction by asking a question:
- "Did the guard wake up?"
- "Did he have his sword with him?"
- "Was it a *talking* horse?"

The narrator can decide whether to answer positively or negatively to each question, and thus whether to incorporate those elements into the story:
- "No, the guard remained sleeping as he sneaked past…"
- "Yes, luckily he'd remembered to bring his sword, and drew the blade…"
- "No, it was just a plain old mare. It stopped and started to chew at the grass, farting gently…"

I want there to be a cost involved in steering the story, but I can't decide how best to do this.

Version A is that every time the narrator says 'yes' to one of the other players' interruptions, that player has to pay the narrator a token. In other words, the players pay the narrator to make things happen the way they want it. I'm guessing that the narrator will find it advantageous to introduce more hazards and conflicts, which the other players will then pay him to get out of. At the same time, if the narrator says 'no' to one of the other player's interruptions he has to forfeit a token, into a central pool. That stops him from going off the rails and narrating something completely against what the other players want. The pool of tokens would be divided equally amongst all the players at the start of the next scene.

The alternative Version B is that there is no cost for the other players interrupting and making suggestions. The narrator is expected to say 'yes' to all interruptions and incorporate them into the scene, but that he must pay a token into a central pool if he says 'no'. I can see that doing it this way would prevent the narrator just trying to hose the other players agendas to get them to pay him tokens, but I worry this would have the narrator effectively mobbed by the other players.

Thoughts on which, if either, works better?

I haven't played either of the games that are inspiring you, so maybe there's something fundamental I'm missing about the idea. But it feels like to me, what's missing from this is some sense of purpose for the various roles, narrator or player. Does each 'player' have a stake in a particular thing happening?  As in, they're designed to root for a particular character, or a particular outcome to the story?  Or are the players just sort of along for the ride? What the stakes are in the game and what the point of it is, other than telling some kind of story, might help you to better understand what associated costs there should be for a particular action.  It's not that the game has to have a win/loss condition, but it seems offhand like there's no real strong reason for a player to participate, token costs or no token costs.  In that case, Version B seems more appropriate, as long as you're dealing with a small amount of players... since it doesn't really matter where the story goes, anyway.

Hey, Mike! Sounds interesting!
Option B sounds like it more closely resembles the dynamic of a standard roleplaying game with regard to the narrator/PC relationship - sort of a "Yes, or spend a token" constraint for the narrator. I'm curious as to what it would look like, given that a more easily-trumped narrator would probably be narrating scenes that didn't focus on his own character or some such thing.
Vaguely, it reminds me of Polaris, as so many things do: in that game, you've got an active protagonist, his antagonist, and 1-2 players portraying neutral or friendly NPCs. Each scene swaps all the roles to other people. The equivalent of spending a token is "exhausting a theme", in which you tap into a specific trait, fact, or resource owned by your character, consider it "spent" for the time being, and thus have the right to say "no" to someone else's idea.
I imagine in Option B, though, you'd have a standard "The GM and the players" setup, only the GM role changes around from scene to scene, but the token economy could work similarly.
Maybe, to keep the flow of tokens going, players have to pay a token to say "No" to something the narrator wants to do. If everyone abides by "Yes, or spend a token", it's basically your standard rules of improv (almost - standard improv is a "Yes, and..." attitude, with any hard "No" actually derailing the process. Here, it's different). Thoughts?

AJ, Abkajud,

Thanks for your comments. Abkajud, yes, switching it round and having a cost for the player's saying 'no' is interesting, but I think AJ is right that this is a mechanism in search of a game. I tried to flesh out a little what I want the game to do, and discovered that it doesn't quite fit with this mechanism, though perhaps a ghost of it survives. Just about to start another thread on that game, I'd be interested in your thoughts on that too.

Michael.

Mike,
 One way to do it is to have a resource. The narrator gets a resource for saying Yes and gives a resource to say No.
  The point is, if there is a narrator that is always blocking, they will run out of tokens and have to say Yes. And if there is someone who always wants to add more to the story, they will run out of tokens and have to listen instead of interrupting.

  A good example of something like this is ...In Spaaace! I have played it and it was very rewarding for a diceless system.

  I hope this suggestion helps.

You might also look at "The Princess Game" by Colin Fredericks. In it, players take turns narrating short statements limited to certain areas. For example, one player represents the bad stuff that can happen to the character, while another represents the character's good heart. Jumping in with your narrative costs you some tokens.

Polaris divides authority over different kinds of trouble in a similar way.