JasonClearwater and I ran a collaboration recently
where we each took an already established character from our own worlds, and threw them together, in sequence with each character's established story arc. We then wrote turn-and-turn about maybe 1000 - 1500 words at a time, each writing off whatever the other wrote. No pre-conceived plot or outcome in mind, no notes to each other than "ok, your turn".
It stretched both of us as writers, and was only my second foray into GM, so that was both intriguing and interesting, to say the least - the recent thread on seduction comes to mind! Not a massive response but it pulled up a Red H, so pleasing enough.
We both found a different intensity came from writing that way, because neither of us knew where the other would take it. It's not for the timid - given as we threw a hetero and a bi writer together
Jason can add his own twenty cents worth.