Maxperson
Morkus from Orkus
A DM under no circumstance is allowed to improvise. They are not playing the game, they are running it. It is the players who choose where to go and what to do by telling the DM how to move their pieces around the hidden game board. And just as in any game anywhere a player cannot "ruin the plot", aka act against someone else's wishes, when playing a game. They are expressing their own desires sure, but really they are attempting to score points in a game.
Improv is probably the biggest and most important DM skill. Without it, the players don't have unlimited things they can try, they only have the small handful listed in the book.