Could be a lot of different things, but ill try and give some examples.Maybe we can drill down a little here. When you say impromptu establishment of fiction could you be more specific? Maybe a couple of quick examples about what kinds of details you mean, and in what context the 'impromptu-ness' takes place?
It might also be useful to get granular about what you mean my exploration in this case. It isn't a controversial word, but is can get used in more than one way vis a vis RPG play.
I'd like to engage with you here, but I want to sure I'm not assuming anything about what you mean, if that's alright with you.
- I really enjoy environmental storytelling where elements of the environment, can be missed, but if paid attention to can hint at secret narrative truths the players themselves can deduce. Player establishment of the backstory of the campaign world, or additions to descriptions of the environment can functionally 'contaminate' the text of the game world, things that could otherwise be hints could be subverted by additional lore established during play.
- I really enjoy strong cause and effect dynamics in my worldbuilding, it is both in line with the themes of the world and a result of a carefully thought up history. The lore elements as suggested by players at the table don't tend to respect the same standards, I have my own pet peeve name for this "elven village syndrome," where the player will neglect preexisting lore to plop down something convenient for say, the needs of their backstory. Another player declared themselves to have been a slave in a nation run by angels that doesn't have slavery. Neither the original elven village person, nor the slave character person were much interested in delving into the lore to fix these discrepancies, it was in their way. I leave the Halycon city of Masks undefined to facilitate this as the book heavily emphasizes Halycon as being a space defined primarily by the needs of the players, but well curated lore to discover and internalize is a big draw for me as a player as well. Our current Masks game manufactured a river district torn apart by demons, entirely through character creation.
- I like the sense of 'uncovering' so when, as a sidebar in the 4e DMG 2 suggests, the GM mentions a tower, and a player chimes in that "are there tentacles coming out of it?" And the GM goes "Yes and..." I become aware that elements of the fiction are being generated without that cause and effect i just mentioned. I'm reminded of a creative writing club exercise in undergrad, where we would each write a line in a story, with each person adding more to the story of Mildred Milquetoast, and then see what we got in the end, projected onto the structure of a roleplaying game. I don't actually mind this in moderation, but that means layering it onto what is functionally a "story before" setup, rather than "this is where all the fiction cones from."