Both. Either. Everything. All. 

I have some posts I've done such comprehensive notes on that it's absurd, and others that I just wing it on. When I apply for a character I'll always have at least a dozen roughly sketched out arcs for them that I can use and know I can write towards. But things change as the IC and game progresses.

Many times I'll just write out posts, entire scenes and dialogue, in my head as I try to sleep at night. That's how the last Doom Patrol post was made, and how the next one is being drafted, as well. Written in my mind as I turned off the lights and closed my eyes for four nights in a row. Then just typed it out first chance I had and smoothed out the rough edges.