RogerD
SOC-12
In the event, I made it work but character motivation best comes from the player and if you write a one shot for specific characters, you should probably think about the character motivation and maybe give suggestions when it might not be obvious.That's where coming up with a motivation comes in. Why would you leave your flock? Maybe you were disgraced, or maybe you betrayed your flock in some way, and now you seek repentance.
You were playing a one-shot so you have to play a character that fits the adventure, but you could spend a minute or two to give some backstory to the character. As rlmercad069 points out, even Shepherd Book had his reasons (and his story in the Pilot episode surely didn't explain ALL of his reasons).
I would bet writers of Firefly spent a lot of time sketching out character backgrounds before the pilot filmed. Not everyone could go from “village priest” to “Shepherd Book” in the first minutes of a 4 hour one shot adventure at a convention.