You're assigned another author participating in the ficathon, but they don't approve or collaborate. Well, I suppose they do in a way. By signing up for the ficathon, they're agreeing to have their stories be remixed. But I don't have to let them read the story before I post it or anything.

The participants are matched by fandom (i.e, you and your chosen remixer will have at least one fandom in common), but I got to pick the story I wanted to remix. Each author can have one safe story, which means it's not allowed to be remixed. But other than that, it's all fair game.