I love collaborations in general, so I very much tend to the player/player kind of style. I usually have SOME idea of where the story is going to go- not necessarily an end point, but a goal for the characters to work towards at the start. Both of us contribute plot points, characters, etc.
Sometimes there’s a lot of discussion beforehand, sometimes there’s not. During the stories there is always OOC discussion (in a separate thread) as to where we’re going.
The biggest thing is finding partners that I mesh with, I think.