Only have one RP hosting credit to my name, but I'm very in on the "create situations, not plots" school of thinking. I set up some locations and NPCs, added some events to introduce both of those to the players, and then everything else I just built from those elements during play. At the same time, though, I don't think it's impossible to have, like, a complete story in mind when setting up, it just needs to be flexible enough to change when players inevitably run roughshod over it. From my RP, the major steps are pretty set; it's a journey and I already at least have an idea of each location on the way. Even the finale is rather concrete. Player expression, then, comes from how they resolve the situation on each stop.