It's the eternal dilemma of GMs. There's never enough time to completely prepare for the next gaming session.

So how much preparation do you need?

When I first started role playing games, using AD&D, I followed the standard template put forth by modules. I mapped out dungeon complexes in detail, placed each monster, trap, treasure, and NPC. I sketched out cities at the block level and figured out where the main locations were.

I created stats for all the monsters and NPCs, and worked through what would happen in the plot if the PCs took some particular action.

I was prepared!

Over time, as I started to realize what made a great game, I prepared less and less. My maps became a bit more sketchy, the NPCs a bit less complete in terms of stats, and I worried less about making contingency plans for every single action the PCs could take.

I had started to enter the realm of on-the-fly role playing.

I first realized this when I looked at some of my AD&D campaigns. In my mind, I remembered the full breadth of the campaigns, and the details that emerged during play. They wove together in my memories into a detailed tapestry.

My campaign notes, on the other hand, were little more than a paragraph for a plot summary, and a map of a location with some NPC names written in different locations.

This minimal preparation had a lot of advantages for me. There's nothing worse than putting hours and hours into plotting out an adventure, only to have the players do something that derails it in the first minutes of play. Or to spend hours lovingly creating stats for an entire dungeon that the PCs manage to creatively bypass.

So I spent my time creating what I absolutely needed to create, and allowed the rest to flow during play. This approach isn't for everyone, I admit.

But when you get to the point where you can do it, you'll find your players having a lot more fun. The game world seems to be far more responsive to them, and their actions have a definite impact the campaign setting. Creative players may twist the plot in ways you never expected, but since you didn't have an investment in the detailed plot anyway, you can relax and enjoy the ride.

This isn't just about allowing the game world to react to what the PCs do, but is about creating the an exciting story around what they do. The PCs are the heroes (or villains) of the story. Everything they do, by definition, has a major impact on the story.

Do the players pick an irrelevant plot element to focus on? If the heroes are focusing attention on that element, make it a major element. Sure, that changes your plot, but your players are left feeling like they're pivotal elements in the story, not just stumbling around trying to discover your plot.

These days, my preparation is typically a page of text describing the background of the story, the main characters involved, and important locations. During play, I invent other locations and characters as needed (my players have gotten used to me writing down the names of NPCs they've encountered). The story evolves around the PCs actions.

My players have learned to expect this level of responsiveness in the scenarios I run, and I've learned to enjoy the lack of preparation. Each time I run a scenario, it's a bit like I'm stepping onto a stage, but once play starts everything flows smoothly.

So the next time you have an adventure to run and not much time to prepare, consider creating the details of the story on-the-fly, during the game. Don't tell your players you're doing it, and they'll probably never guess!