In a private on-line discussion I mentioned that I felt GMing Amber was less a matter of GM fiat, and more a matter of "popular opinion." I felt the comment warranted some further discussion and explanation.
I probably should have said, "Popular opinion navigated with an iron fist," to allude to my first Amber GM's experiences as well as point out that GM fiat can be necessary to direct a game. A GM needs vision, and while a narrow vision will railroad players, a consistent vision is necessary to keep players from getting too far off the map(s). There be dragons there, my friends, and while I approve of a dragon slaying once in a while, they're not always in season.
This is not limited to Amber GMing, of course. The difference between GMing Amber and a number of other games is that the qualities that make a good Amber GM are almost contradictory to what makes the suspense of your average Amber campaign. You need to be able to trust your GM. You may not be able to trust anyone else, including yourself. (Or was that Paranoia? No, I think that's still Amber. In Paranoia, you don't trust the GM, either.)
I described briefly a conversation I had with a gentleman in a gaming group of which I was once a participant, and I explained then that the appeal of diceless was that you focused instead on the events upon which using dice would have made awkward, broken the mood, or otherwise changed the concentration from the events unfolding to the mechanics. (There's a phrase for that sinful act, I'm sure, but I don't hang out in that many metagaming forums.)
I think the question continues, then, with, "How do you make your decisions as a GM, then?" The implication, of course, adding on the words, "...and remain 'fair'?" Now, without going into my hypothesis that no player ever really wants you to be "fair," the answer for me has been "popular opinion." The events happen the way the group desires.
This is a broad rule of thumb, but gaming is generally not an activity of individuals. (This is one of the things that makes PBeM different from FtF...the amount of feedback you receive as a player.) Resolutions need to be consistent to the needs of the game. Not to "the plot," not to "the system," but to the game itself as an entity that is fueled by the combined imagination of the players and the GM.
We create legends for our characters in any game. The bad rolls of Thorn, the Dwarf Fighter, constantly fumbling and injuring himself. The 100' radius of death Benedict wields. The way Vebastian growls as he plays the violin. The fact Aziz, "doesn't really like girls." There are other reasons to play, of course, but those legends are significant and because they've grown as a symbiotic part of the campaigns those are the first place I go in making decisions to what happens. If a huge game-making fight happens, and Thorn doesn't manage to nearly lop off a limb...will Thorn's player be disappointed? Or does it become, "The Exception!" that Thorn excitedly tells his cleric-friend about almost every time she asks about a battle?
That's my job as GM: to figure it out, to describe the dice roll with, "As his axe swings dangerously close to his own foot..." We're in this together, and we want a game we all enjoy... so what happens isn't necessarily just what the GM wants: it's what everyone likes.