Making up all the campaign background for a large-scale game is a daunting prospect, and there will always be some details that no GM will have time to fill in. This is where you can use the players. If they say something while chatting in character that adds to your setting... write it down!

The converse is that if they say something that you don't want to be true in the setting... write it down and see if you can twist their words somehow. You can get some truly devious ideas this way.

Transcript

C-3PO: Celery. Apples. Walnuts... How are we supposed to get grapes on an ice planet?
R2-D2: Peeled eyeballs would do.
[SFX]: < po-poppy bibuzz ding-bop bleep >
C-3PO: We droids are the ones holding this Rebellion together. But do we get any respect?
R2-D2: Negative!
[SFX]: < boop >
C-3PO: They tell us that once we overthrow the Empire droids will have equal rights. Meanwhile... "Get that!" "Fetch this!"
R2-D2: Right. The Empire eliminates meatbag slavery, but droids... noooo...
[SFX]: < whir doing-whree boodit ding buzz doop boop boodit >
C-3PO: Yeah!
GM: Um... <scribble scribble>
C-3PO: He's writing it down!
R2-D2: Ooh, ooh!
R2-D2: But that time they upgraded all droids with plasma cannons, that was awesome!
[SFX]: < doing-whree fobuzz doop-whree ta-poppy whroop bee-OO-oop >
GM: Nice try.