I’m one of those GMs who runs a reactionary game. I ask every player to devise some kind of goal before trying to determine a story outline. I’ll develop a game plan for a session once the first scene has played out and I’ve got an idea of where the group would like to take things.
Don’t get me wrong, I like to organise things. I’ll prepare a setting, with an assortment of locations ready action, a rogues gallery of NPCs ready to throw into scenes and a few potential treasures or maguffins. In some circles, I think my GMing style is called “playing unsafe” but I prefer to think of it as allowing players to actually make decisions for their characters, providing the opportunity to explore the setting and explore the characters within that setting.
As a player, I’ve participated in sessions run by similar GMs and have thoroughly enjoyed it. I’ve also played in sessions run by GMs who prefer to simply run things according to a set scenario with no allowance for player decisions to really affect the story flow. Sure, the players can stop, go, speed up or slow down the course of action; but with no chance of taking side routes or simply dismissing anything not related to the GMs story at hand, it’s all just railroading. I’ve delved into a bit of this with my various posts on Vector Theory.
So, there’s nothing really special about this particular method of spicing up your game. If you’re a GM, just allow your players to make decisions, don’t be afraid to follow where those decisions might lead the story. If they lead to new and interesting places, keep notes. If they peter out, go back to the planned story you had, and incorporate some of the exploration that the players took you on while they were leading things.