Every current or aspiring Game Master needs to face the reality at some point that their role is not to simply make the player’s lives the absolute worst. This is unfortunately a difficult thing to convince some people. It is a vital and necessary thing for Game Masters to be adaptable to whatever the players decide to do in a game, be it a quick adventure or prolonged campaign. But I would argue that there is a time to not be flexible. A time to dig in and double down on decisions already made and to not succumb to the seductive allure of making an adjustment just to upset the players and assert some measure of control over the direction the game is headed in. And when is the time for being inflexible? When the villain has already made their plans and settled in on a course of action and the players figure out a way to outflank them and emerge victorious. That is when a Game Master needs to step back and let the clever players take a victory lap and savor the sweet taste of success.
The reason for this is that sometimes a villain just isn’t as clever as they think they are. Any Game Master can move the goal posts willy-nilly and insist that the players need to keep working against long odds in order to keep their heads above water. This is guaranteed to entertain the GM of course, but the players are going to get pretty damned sick of it. The purpose of Dungeons and Dragons is to have a good time and that means for everyone. So the GM has designed a great villain who has one or two nefarious plots cooking that will upset the world in which the players are immersed. Take a cue from every James Bond plot: no matter how terrific a plan the antagonist has designed, a determined and resourceful hero (or group of heroes) can and should triumph and defeat the evil. So my plea to all GMs is simple and I believe agreeable: let the players find a way to best their enemies.
Now this isn’t as easy as one might think. Some villains have magical access to information that might cause them to adjust their plans and foil players. Others might even be able to warp time itself to cause the players innumerable headaches. A GM must not give in to these temptations. Allow the villain to be over confidant (at least to begin with). Cause them to set some of their plans in stone so to speak, and not allow them to alter them after a certain time. And finally, be honest about what information the villain might have access to. It isn’t feasible for a master nemesis to be eavesdropping on the players all of the time even if it is possible. Not everyone the players talk with is a spy. Sometimes the players are just so ridiculous that the villain doesn’t take them seriously. Let the players have some fun with their own plotting and conniving. Incorporate their ideas into the story and if it works to solve the problems that they encounter then all the better.
The other side of the coin is that any villain worth its salt will subsequently adapt to deal with opponents that best them once or twice. This allows a GM to make their villains tougher and more troublesome as time goes along. A villain that escapes to plot again will be more of a challenge later on. So being inflexible can eventually lead to being more flexible, until the players can come up with another good plot of their own to come out on top. And then the GM has to once more let them take a well-earned bow in the spotlight.