100% Spontaneous.
I dislike having Biographies and Personality sections of character sheets. Feels too restricting, and if I end up going against the CS, then what was the point of having it at all? Biographies. Not sure how it makes sense that other people know this characters backstory, I would prefer to reveal my biography if it makes sense in the context of the story.

With GMing, I do planning as in "okay this is the world, it would be cool if i could work this into it" and thats about it. Planning for me, just kinda sucks the sense of adventure out of it. I'm no longer involved in the world, not exploring it with the players. That's another thing, I love it when players help build the world with me. I always find it odd how, say, in an open world fantasy RPG, characters who were born and raised in the homeland ask questions about culturally traditions that I came up with. Lame. Let's work off of eachother to get this ball rolling.

Most people keep writing and roleplaying very tight with eachother, but I personally like to think that roleplaying and writing are too completely different things.