In spinning out a comment on Rob Donoghue's Some Space To Think, I mashed together coding concepts and GMing. And it got me thinking...
Design-Time GMing covers everything written up before the game. The monster stats, NPCs, traps, treasure, maps, overall adventure design, and side quests all fall under Design-Time GMing.
Run-Time GMing covers everything that happens when the players sit down at the table. Rules arbitration, interpreting PC actions, playing the NPC roles, having the environment react to the characters, and making the world come alive all fal under Run-Time GMing.
Why is this important? Because you need both if you want to be a really effective GM. There's a line between the two, but tasks on either side of the line tend to bleed into each other. Consider players who tend to ignore the planned adventure in favor of doing things their characters find rewarding. If you're a Design-Time GM, they would drive you insane by actively avoiding all of your prepared notes. If you're a Run-Time GM, you can keep up with them and pick random things out of the air for them to do, but you'll never be able to give them the depth of story that can come out of Design-Time preparation.
Here's where both sides meet up. As a balanced GM, you need to have enough material prepared so reacting to your players going off script doesn't derail the game. If you have a couple of side adventures prepared and some hooks embedded in those side adventures to lead them further down the path of your game's main story, all you have to do is react fast enough to hide the plot seams from the players during Run-Time.
So many places put emphasis on one side or the other of this equation. Let's figure out other ways to come at it from both sides at once. What's your best trick for running a game, and how does it fit into the Design-Time vs. Run-Time spectrum?