A new writer joined The Fiction Workhouse a few days ago, a man who has won quite a few awards already for his short fiction, but who wants to strengthen his game.
He read a very short throw-away flash of mine on site and posted a riff. I responded with another... same characters, same voice... just extending the story's scope. Two brains working together on the same thoughts but taking turns. The story went through various twists and turns... until between us we has a few thousand words, slightly disjointed some of it, but very, scarily coherent in other ways.
The whole thing sudden, unplanned.
My pieces were from a man's point of view, his from a woman's. They 'replied' to each other, echoing the content of the flash before, extending the idea in others. Fascinating!
The images if you dug them out, got more and more raw. There was unstated sexuality in the piece to begin with and it got stronger and stronger, darker and darker.
When we'd finished, we both went away and edited, turning the piece into first draft short stories.
Thematically this was a really fascinating exercise. While we'd been riffing and continuing a story set by someone else, our own themes were coming through. At least, mine were. I ended up, after a lot of tidying, editing... with a potentially strong story.
Try it... a great exercise, invented out of the air.