Whose theory?
That's the whole point of my post, which is an elaboration of one aspect of what (I take it to be that) [MENTION=5142]Aldarc[/MENTION] is saying. Some people like to play a RPG in which the GM decides everything that happens except (perhaps, if there is no fudging of the combat rules) who gets beaten in fights. Others don't. And it's hardly a new idea. I already cited Classic Traveller which has rules which allow the players to declare actions which result in NPCs acting one way or another. Moldvay Basic also includes reaction rules, and examples of the players declaring actions which are intended - as mediated by those reaction rules - to determine the behaviour of NPCs (some hobgoblins).
Both Classic Traveller and Moldvay Basic also have morale rules, which are another set of mechanics for determining NPC behaviour.
I'm familiar with morale rules, though I've only ever used them as guidelines for NPC (usually opponents, sometimes henches) actions rather than hard-and-fast determinants.
Moldvay Basic is a version of D&D that I don't own. I've got regular Basic, and all of 1e through 5e except 3.5.
As a general rule it is the purview of the GM in the same way that it is the purview of the GM to decide what a NPC does in combat.
But if a NPC is dead, the GM is bound by that. If a NPC is subject to a Suggestion spell, the GM is bound by that.
Yes, and the same is true for players with regard to their own PCs - players are bound by such things too.
If a NPC is persuaded by a PC, the GM is bound by that.
And here we run aground, because an NPC can't persuade a PC in the same manner without use of spell or magic to back it up. The player is not bound by any persuasion (or intimidation, etc.) roll from the GM, if the GM is even allowed by the rules to do so. To me, this is wrong - if it doesn't work the same for everyone in the fiction then it doesn't work at all.
Thus, if the players were bound by GM-side persuasion rolls then I'd be quite happy with the GM being bound by player-side persuasion rolls. But I'll quickly point out that such a system would be too constraining on player agency over their PCs; and so the only way to make things consistent is to unbind the GM's reactions to match.
That said, a good GM would usually still take any such rolls into account as guidelines when determining a reaction - but would not be bound by them. In the Duchess example, there could be rolls on both the player side (to persuade) and the GM side (general disposition), with both informing but not dictating the Duchess' reaction and with the GM best off not knowing exactly what reaction the PC is trying to get.
*** - note that in the above, the word 'persuasion' is generic; feel free to substitute whatever equivalent social mechanic term makes sense for your system.
The whole function of mechanics is to constrain, and establish, the content of the shared fiction.
Or to get in the way, depending on circumstance.
What do you mean by "the odds" here?
We were talking about a situation where mechanics were in use to help determine the NPC's disposition toward the PC; mechanics mean dice, dice mean random chance, and random chance has odds that can be influenced by in-fiction actions - in this example these actions might include information-gathering, perhaps a little espionage, and so forth in order that the PC's actions and words at the time are most likely* to get, say, the desired reaction from the Duchess.
* - 'most likely' still does not mean guaranteed.