In a pen & paper roleplaying game like Dungeons & Dragons the DM's role has two major parts: Preparing the world and adventure before the game, and improvising the details during play. Normally the sequence of these tasks is going just one way: Adventure design comes first, and what is written in the adventure then is the base for what happens during gameplay. But as I discovered in my current campaign, playing the sandbox adventure Madness at Gardmore Abbey, sometimes events during gameplay change the situation to an extent that process becomes iterative: The adventure is the base for gameplay, and then gameplay developments necessitate changing the adventure.

As I told in my campaign diary, in the previous session the players opened the door to a boss encounter, only to then decide to run away. If this was a computer game, the next time they opened that door they would find the situation unchanged, because a computer game doesn't usually have this iterative adventure design mode, and the actions of the players don't change the adventure. A pen & paper game is more flexible, and more based on what would be logical, and not forced to stick to a rigid script. So I went back to the drawing board: Why is that boss NPC in that location, what is his motivation? How would he logically react now that he found out that a group of adventurers is around?

Of course this "logic" still is more a fantasy logic than a real-world logic. Who would want to live in a dungeon in the first place? I can't just decide that the NPC in question decides to leg it and take a holiday on a tropical island instead of sticking around in some catacombs, as reasonable as that might be. The NPC has a specific role in the overall adventure, and I can't just eliminate that function from the story. In this case the overall adventure is about collecting the cards of the Deck of Many Things, and the players already know that NPC has cards, so if he fled it would be impossible to finish the adventure at this location. I could then force the players to give chase, but as the adventure is already huge, I don't really want to extend it much more.

So it is more a question of modifying the encounter with the logic of "forewarned is forearmed". If an NPC lives at the end of a dungeon he might consider the monsters of the dungeon as his lines of defense. Once the NPC becomes aware that those lines of defense have been removed, he will rethink his defense strategy. The general idea here is that the players had an advantage of surprise, and that by running away they lost that advantage. And how the encounter plays out when they come back needs to reflect that logic. You can't just announce your presence to some villain NPC and think that won't change anything.

This was just one example, but there were other changes to the adventure I did based on the actions of the players. This is quite fun and interesting. The adventure grows from a script to a living world which reacts to what the players do. That is especially helpful in the context of 4th edition being encounter-based, and the actions of the players helping to flesh out the connection between those encounters. In the end you get an adventure which is unique to your group, and not a copy & paste of everybody else's experience in Gardmore Abbey.