When I say "your friend concedes them authority over the ending of their book" I mean chooses to imagine what the author invites them to imagine. I should have made that clearer as you have both understood that concession as letting the author write what they want, while imagining something else. I'm not sure how you got there, but that wasn't my meaning.
By what means is it shared, if not externalised? That said, I'm locating objectivity at least in part, in there being a process or party able to add description. Based on
@Manbearcat's latest, I would clarify that the description I have in mind is not solely aeshetic: it includes that actions may be addressed toward it or incorporate it, and saying how it may respond or be changed.
If I read you correctly, you both want to deny the notion that
Seeing as you likely don't want to deny my testimony to my own experience, you are saying that this doesn't work for you. I'm not sure if will change that, but when I say "decided" I include actionability although I diverge from
@Manbearcat in also caring about emotive and aesthetic qualities... whether or not they are actionable.
Do I read your comment correctly as proposing an alternative explanation intended to supplant my proposal, so that "lots of interconnections" is all that counts? I agree that a web of interconnections is significant, albeit I would say that again, the two explanations are not in conflict. Above you say that each participant's imagination acting as some sort of constraint on the others
won't give rise to objectivity, but that would seem to be exactly the sort of process that would create a web of interconnections... meaning that if your alternative explanation is correct each party's independent and interdependent capacity to add to the web under some sort of constraints would give rise to objectivity. It's hard to see how constraints are not external factors (the constraint of rules, the constraint of what others have said.)
One example that seems salient here is
Ironsworn. I've recommended it many times, and a fascinating way to play is coop or solo. Solo-play provides us with a handy case that rules out the role of other participants in the process. What is noteworthy in Ironsworn - widely commented on - is the role of Oracles in inspiring what you say (to yourself). What I note about that is the work done by the external factor, that makes what you say go on to feel more objectively real to you. I would urge you to play Ironsworn solo and revist the notions we're discussing here.
Rules are an external means for making content more objective. Nowhere am I prioritising one participant's narrative control over another. If a player declares that a guard at this gate is their friend, and I can ask them what her name is and they can say "Jo", then that enhances the feeling for me that the guard is real. I agree with your thoughts about enmeshing, and take them to complement or be part of the reifying of the imagined world.