Ready, Freddie? The eternal question.
No matter what style of game you play, figuring out the proper amount of preparation required to run a session is not a trivial matter. Some modern games make it easier on you, providing you with tools to create a session on the fly or else making preparation unnecessary / verboten. The "powered by the Apocalypse" games require a certain amount of preparation early on, but for the most part GMing those games is about reacting dynamically to events as they unfold. That's a fine thing, and a good philosophical perspective on the GM role (that is, the GM's job is not to plan a story, it is to respond to the decisions the PCs make); it can be unnerving if you're not on the ball, though, because you don't have the security blanket of pages of notes to fall back on.
My friend Rob recently ran both DRAMASYSTEM and APOCALYPSE WORLD, and both times he complained a bit about the lack of preparation involved. For those of us old guys who came up playing the traditional way -- making maps and writing pages and pages of ideas or "set piece" scenes -- there is something that is not just discomforting about not investing a lot of downtime in preparation for the game, it's disappointing. We miss that personal investment in the game, that sense of putting a lot of ourselves into the unfolding campaign. Sure, we're putting in the same kind of active effort at the table as the other players are, but it's not the same kind of pleasure. Even if you're not a GM that takes a firm hand with story in your game, having the game buzzing around in your mind during the week, scribbling down brilliant ideas and gut-wrenching developments, that's a real part of the pleasure of GMing for a lot of us.
That's not a knock on those games, there's just an adjustment that a GM has to make when taking on a new style of play, just as a GM that is approaching a sandbox-style game has to adjust to the workload of a living campaign setting where players could (explicitly!) go in any direction at any time.
My wife ran several games of PRIMETIME ADVENTURES over the last few years, and although they were fine games, I think she never quite found the sweet spot for herself in terms of what level of prep was required. Her sessions rarely dragged, despite her being quite open about the fact she hadn't done a lot of preparation (which, I think, was partly thanks to a high-functioning troupe of players in addition to her highly-formed skills at being "present" in a scene). The problem seemed not to be that the games were suffering a lot because of slim amounts of prep time, but that she always seemed to feel disconnected from the games she was running. She never had that feeling of investment and ownership that Rob and I talked about. I know some people are probably tying themselves in a knot reading that sentence, grumbling about how GMs don't "own" a game, but in a real sense GMs tend to do a lot of the work in setting up and and maintaining a roleplaying campaign. A GM that doesn't feel at least a little like the game is "their baby" might have difficulty putting their best efforts into it, or at worst, allow a game to fizzle.
This happened to another friend, Amanda, with a superhero game she was running. She had gathered her players together and done a session of campaign creation, where the players all contributed ideas toward building the setting, which I think is a terrific technique embraced by a lot of modern games. The problem was that the players took the game in directions that really didn't interest her, and eventually it fell apart because she didn't know where to go with it. This is a real pitfall of collective efforts at creation - unless you bring enough of your own ideas to the table, the game can turn into something you have no creative investment in, and then you have a real problem. GMs need to hold their own ideas lightly and be prepared to modify them, but they also need to remember that they are players too -- they are as entitled as anyone else at the table to make the game their own, put their stamp on it.
So what does prep look like at my table?
Really, it depends on the game. Some of the games I love the best are ones that demand a lot of research and downtime development, like my Victorian vampire slayer game, SUNSET EMPIRE. Knowing what that world looked like and keeping it alive in my imagination required work, and that work allowed me to feel comfortable when I came to the table. Sometimes the security blanket is important, and it gives you the confidence you need to improvise boldly when you're actually at the table. Having a pile of notes and maps and ideas doesn't mean you have to use them, and as long as you don't feel like that time was wasted, it's all to the good. Knowing when you need that investment makes all the difference.
Sometimes preparation is about immersing yourself in the genre, reading or watching media that inspire you, listening to the right kind of music. Inspiration is definitely a crucial part of preparation.
For a lot of my current games, I have been doing a fairly low level of preparation. I tend to come to the table with a few ideas scribbled down for each player character -- they aren't quite Bangs, but they're pretty close to that. Sometimes they're designed to put pressure on the character to pursue their motivations and goals, sometimes they're meant to put a bit of dramatic "meat on the bones" of the world -- allowing a PC to interact with someone in a new and unexpected way, or adding a little "slice of life" to the world. These things make a game live, and they can develop in very interesting ways. Sometimes you see something unexpected emerge from a PC interaction with some minor character, and that's solid gold. Players get into their comfort zones, and often hone in like a laser on their goals or their "attitude". Breaking them out of a routine is important.
Tools can help you with this, of course. Having things like a list of characters, props, and locations you can drop in whenever you need something makes it feel a lot less by-the-seat-of-your-pants. Most of us aren't that good at improvising "colour" on the fly. For my OVER THE EDGE game, I kept a few pages of ideas for small scenes of "street life" and open-ended story points that the players could engage with if things slowed down. That game was largely improvised, and even if I didn't get to run it to its conclusion, I felt like most of its wild, weird ideas actually connected and made sense to me. My players might have thought it was random weird bullshit, of course.
Sometimes you need to put in more time, though. If you know that Important Shit is going to go down this session, you need to be ready for it. Any mechanical preparation that's necessary (such as statting up an enemy, or understanding a particular mechanical subsystem that might come up) needs to be done and handy. If you know an important scene might play out between two characters who have a lot going on, you had better be ready to get inside that character's head. Fumbling for ideas when the big moment comes -- unless it comes along entirely unexpectedly -- is bad form. If you know that this week is going to be a Big Climactic Episode, you had better have all of your ducks in a row, and that might include rough breakdowns of scenes that could happen, even if they don't play out the way you expect them to (and they won't). When things start to happen fast in a game, preparation is what makes them happen smoothly.
Running a con game is a whole different art. Unless you're playing something like FIASCO, where the toolkit is the whole game, you need to keep it tight. That means having everything you need at hand and an eye on the clock. Sloppy con prep usually means an aimless or overlong game.
Being a GM is largely about being aware of what's happening in a game, both at the level of the scene -- being "present" enough to have interesting things happen in your interactions with the player characters -- and at the level of the session and the campaign. You need to be aware of what's likely to happen in a given session, or what needs to happen in the next few sessions (assuming you've decided that a game will have a set length). Preparation is sometimes art, sometimes craft, and sometimes a pep talk for the GM to give themselves. It is always necessary, to a greater or lesser extent.
Preparation is putting your awareness of your own game into action.