I'm basically letting my player's improv determine campaign world elements whenever possible. Unless I've already defined something, if they come up with something good, it's true.
Here are things I learned from my PCs in our two sessions so far:
1) My Catholic-based church doesn't assume a vow of chastity. Inquisitor Marco cheerfully announced his priest had returned from the Caves of Chaos and was going to partake of the keep's food, drink, and women. I said "Women?" "I don't see a vow of chastity on my character sheet." No, he didn't, so I guess the church doesn't concern itself with that.
2) Goblinoid kids may grow up really fast. Lots of "goblin kids become goblin adults in a week" jokes have made me think that maybe it's true. Maybe they do mature in extremely short periods of time. A week seems short, but months, perhaps . . . they may live in dog-years.
3) You need a hunting permit. It's 20sp for a hunting permit, and then you can pretty much camp around Falcon's Keep and kill and eat whatever you want.
4) Mountain Killer Frogs are big, not like the little squirts you find in the swamps. And Honus's neighbor's tribe raises them.
I'm sure I'll learn more, but it's a lot of fun letting everyone fill in details, even goofy ones, instead of telling them the game world. Don't explore it, make it.