The information flows at the same time and rate as it would if it had been prepared, just from a different direction.
Note that this isn't theory. This is how Fate works if you are starting a fresh Fate game unconstrained by an existing setting. Everyone sits down and collaborates on the nature of the game: genre, setting, conflicts and characters. You can do this exact thing in any RPG, whether something traditional or otherwise. You lean into genre to fill in space that have not been defined yet. If you are playing a game in the vein of Arthurian Romance, for example, the things that are true in Arthurian Romance are true until they are not true. That gives everyone at the table purchase. They are speaking the same language from the same tropes and assumptions, until someone declares something that is different from those things. You could define your genre as "The American Old West but Dinosaurs never went extinct" and do the same thing because people at the table (presumably) understand the tropes surrounding the Old West and Dinosaurs.
I do think either a light ruleset or one with which everyone at the table is very familiar is best for this sort of thing. it isn't impossible otherwise, but it moves more smoothly if less time is spent trying to model things in the system.