But this just comes back to the role-playing vs. being a author conversation.
Gandalf had no say in whether or not Bilbo found the one ring. When he showed up at the beginning of Fellowship and threw the ring into the fire, he was inside a story controlled by facts outside of himself. If he had suddenly starting talking to the reader and announced that he decided it was not the one ring, that would be a very unsatisfying development.
Same thing for whether or not the brother was possessed willingly.
Obviously a DM and players may plan outside of a game to agree to certain truths. At a macro level this happens when the group decides to play D&D over Mutants and Masterminds. If a player wants part of the plot to be that his brother was possessed against his will but this truth is not generally accepted, then this is fine. But ultimately a great deal of pre-authorship is still mandatory for the experience to model "being that guy in these circumstances". If the player can keep changing the rules in media res, then the resolution is completely divorced from the character's capabilities.
There is a great deal of merit to the idea of experiencing a story exclusively as an individual inside that story. The demand for significant pre-authorship in no way prevents players from contributing to the "pre" part of that.
...snip...
Once the story is moving at the table the players are either in the role of characters within a set of circumstances or they are not.
First of all, any opportunity to bring in
DM of the Rings to the conversation should be celebrated heartily.
The question of pre-authoring versus Story Now / Just-in-time GM-ing / mutable fiction is obviously not a binary. In fact, I'm a strong believer that diligent, coherent pre-authoring is a necessary precursor to running a successful campaign. To me it's much easier for the GM to later break that pre-authoring when needed if they have a strong grasp on how a given "break" will spill out into following frames.
The shift to "Just-in-time GM-ing" happens more directly in play. It's a reaction on my part to trying to be more open and flexible to player desires. And I know for me it has worked wonders in building the types of campaigns that I enjoy. My 14-month long Savage Worlds fantasy campaign was a direct result of a dedicated commitment to not having any "end game" in mind, but to "scene frame" based on what the players were giving me, with
just the right amount of pre-authoring to make the frames coherent.
To follow up on the hypothetical Lord of the Rings example:
If I was the GM, the nature of the One Ring would be set in stone. But let's say the player running the "Frodo" character came to me and said, "What if I'm not entirely sure my uncle Bilbo is telling me the truth?" Maybe it's because he wants wants to explore something different in his character than "tragic heroism," so he imagines up that his uncle Bilbo isn't a good guy, but is in fact manipulating him.
The Ring is still the Ring, but now the Frodo character is exploring an entirely different set of fictional circumstances to react to / play against.
Would I as a GM be willing to change the fiction to potentially give the player what he wanted? Prior to 2010 or so, the answer would be a definite "No. That's not how the story is set up." Now? I'd strongly consider it.