Someone called Blackberry posted this over on RPG.net (in a totally unrelated discussion):
We don't know his name -- he invents one.
We don't know where he comes from.
We don't know where he goes.
We don't know his motives.
We don't know his past.
We don't know his future.
We don't know his favorite ice cream flavor.
But there are scant few characters more complete than Sanjuro in his time on screen."
I think this is very, very, very good food for thought. Show, don't tell. It's actions that define characters.
Stuff like "motivations" are tools for the players to make characters move and act (because players often suck at making characters move and act).