But how does the players' sense of it change based on when the GM decided it? As opposed to when the GM narrates it?Here's the thing. A living, breathing world isn't for the benefit of the PCs. Of course from an in-character perspe.ctive it doesn't matter when you did it. It's for the players benefit that it's done in advance and it gives THEM the sense of a living world
The process is the opposite. Random generation determines events pertaining to the PCs - the PCs meet a starship, or a NPC on world, or a particular patron who wishes them to undertake a mission; the PCs find that the demand for the goods they're trading is quite high, or conversely it is very weak; etc - and the GM then establishes fiction around this to give it meaning, if it's not already obvious what that is.What happens once you determine that there are droughts on some worlds, depressions on others and say wars in 3 systems? Where do you go with all of that information once it is randomly determined?
For instance, when the PCs were on Ashar one of them was banished for having committed a crime. She entered a neighbouring country. It had already been established that the two nations were hostile to one another; and that religion was important in the government of both of them. When the PC encountered fugitives, it was easy to present them as religious refugees, adding additional detail to the already-established situation.