New to this System

So I recently acquired the Core Book for the Firefly RPG from my girlfriend as a birthday present. From what I've gathered from my reading and the videos I've watched. This is a narrative driven game where you have to do a lot of prep and in order to validate that prep you have to railroad a bit of a core storyline unlike in DW/D&D that gives you a lot of flexibility when creating story/situations and NPCs. Am I getting that correct or if I'm wrong how do you prep in order to add flexibility?