A couple things for folks who appear to be citing firmed up backstory for the sake of internal consistency and who are decrying the various stages of creatures and the PC:NPC mechanical divide.
1) On backstory
I'm running a (slow-going) PBP for my S.O. on here. There is 0 chance she will read this (no time nor inclination to read these boards) so I risk nothing with spoilers.
We came into this game with very little fixed in terms of backstory. We have a few elements here and there, some ethos-guiding beliefs, the thematic components of her PC build, and the fact that she is from the Feywild and now lost in another world after an encounter with a moonlit encounter with a spectral stag. Ah hell, I'll spoil it just in case.
[sblock]I have thoughts on who the spectral stag is, where she is, and what choices she will have to undertake in the conflicts to come. I'm thinking that the relevance of her Ghost of the Past theme is that she is the restless spirit of an eladrin who died during the final, brutal moments of war against the lycanthrops of Brokenstone Vale (in the Feywild) before they gave up their homeland. From her invocation of her theme earlier, I'm thinking that she was a mother who died protecting her children from the lycanthropes (who in turn surely died). I'm thinking the spectral stag that brought her here is the Archfey leader of the Gloaming Court, the Maiden of the Moon, who is forever locked in a battle with the lycanthropes of the Brokenstone Vale but is bound by oath, and perhaps edicts by the Fey Courts, to abide by certain rules of engagement with them (basically a North/South Korea sort of Cold War). I'm thinking that, in order to get around that oath or those edicts, she is using the vengeful spirit of the PC as proxy. She has found the birthplace of lycanthropy and the means to travel back in time; a lost iteration of the prime world that the gods abandoned long ago after losing it to the Elder Primal Spirits.
The PC will have the potential opportunity to wipe the taint of lycanthropy from all future existence, undoing the horrific future of her restless spirit (and countless horrors for others). However, in order to do so, I will place her in conflicts that absolutely test the ethos cornerstones that she has devised (of most consequence is the law of nature outweighing the law of rulership) and the maternal protective spirit that has manifested in play (ironically, she will have to put the innocent, shapechanger children to the sword in order for her task to be assured - I don't think she'll do it).[/sblock]
This is all subject to change depending on how the game takes shape and what cues arise from her. I think about the only thing that is fixed is that of the identity of the spectral stag.
The fictional world (of course) doesn't exist and, as such, any backstory exists in a state of superposition until its manifested and is confirmed through play. Once it is in play at the table, it transcends that superposition and becomes fixed. Now, my question is, if she reads things differently (her purpose in being there, who she is, where she is) based on the results of our play, wouldn't it make for an internally
inconsistent experience
for the player if I were to shoehorn my preliminary thoughts onto the outcome of play and force her to play out my will (rigid backstory)? Isn't internal consistency only relevant to the surveyor (the players in an RPG)? If the results of our play yield an intuited perception for her that is (somewhat or wholly) askew from my own thoughts, yet I impose the transcendence of that superposition backstory toward my own vision (of which hasn't come about in play to her), what have I gained exactly (except for a jarring and less enjoyable experience for her)? In terms of fun and internal consistency, what have I gained? Fidelity to my originally envisioned model? If so, which is of greater import for the table?
2) On PC:NPC mechanical divide
Referencing the above again, she is in the throes of a conflict on a moonlit bay. As she emerges from the mouth of a river, she finds a father/daughter combo are out fishing in the deep twilight hours. They are attacked by a tentacle beast from under their flatboat.
Now, the immediate dramatic component of this conflict is the protection of the child from the nearly certain death of such an assault. So how is this accomplished. Well, in 4th edition, it is done through (i) minionizing the NPC daughter, (ii) providing her statblock with narrative and mechanical means that fortify her despite her minion status, (iii) providing her protectors (her father and the PC coming to their rescue) with the means to protect her and vanquish their foe that is beseiging the boat, (iv) an encounter budget and monster design that handles the dramatic pacing of such a conflict.
She has 1 HP as an of-level minion. She is utterly vulnerable. However, she gains bonuses to defense so long as she is adjacent to her father, she gains means to make herself untargetable against a singular foe, and some movement ability if things go very awry. Her father gains several means to protect her, including giving her 10 temporary HP as an encounter power and taking a blow for her so long as she is adjacent to him. Now the monster is manifesting round by round as 2 waves of minion tentacles (threatening to pull them off the boat and into the hindering terrain of the water - where they will be chomped and grabbed by the creature and potentially pulled down for a suffocation death) and then the final elite monster (which manifests 4 tentacles twice - immediately and at bloodied) in the 3rd round.
So you've got several iterations of creatures here all working to pace a dramatic fight and answer the question of whether the little girl (and/or the father) will be eaten and/or drown. I don't know any other system that could pull this off. The little girl actually has a chance to survive if the PC can get there quickly enough (she is poling her boat toward them while firing off arrows in between) and perform well enough to dispatch the beast and protect her. Without solid play from the PC and/or some bad luck (eg if the father goes overboard before the PC arrives, things will likely go pear-shaped quickly). I can't imagine how this combat could be pulled off (both the functionality and the dramatic pacing of it) if all of the component parts. The NPC

C divide and the various iterations of a singular NPC in this single fight (the tentacle creature is waves of minions plus challenging terrain and then ultimately manifests as an elite monster) are paramount. I've yet to play any process simulation that can pull such a trope off. The girl would have level 0 defenses, no HP, and no thematically appropriate means to protect herself. It would be difficult for some (but not impossible) to justify the father's means to protect her. And monster mechanics that take the shape of two different iterations in one fight(?); minions + terrain and then a whole beast. Only outcome based design pulls something like this off with any level of guarantee of dramatic tension. The save the little girl trope only works in process simulation if the GM fudges rolls and funnels play toward the dramatic end they seek. It just doesn't manifest as a natural result of play and thus the PC's role in the "dramatic" outcome becomes peripheral.
I'm assuming that whatever internal consistency you feel is yielded by NPC

C homogenization is worth the trade off of a heaping of dramatic conflicts/tropes that have little to no opportunity to become manifest in play?