Here aer some general ideas about what to do, and what not to do as a Game Master for a Role-Playing Game.
Do prepare a general sketch of NPCs. Work out what kind of NPC’s they’ll likely meet in the given situation. Put together general ideas of how they’d react to things. Local town guards are different than big city guards, or royal bodyguards.
Don’t overprepare. Each NPC doesn’t need a script, just basic opinions. Each NPC doesn’t need full stats, unless its a specific direct encounter.
Do get basic maps. Have an ideas of how locations fit together, which helps with travel plans and getting a grip on the locales. You don’t need every crate in the warehouse mapped, but know where the warehouse is in the city, in relation to the PC’s favorite bar.
Don’t get attached to your NPCs. It is likely that your favorite potentially recurring villain will be in the middle of his great expositional monologue when the PC’s sniper caps him. Have the NPC’s lieutenant ready to step up, send in the minions, and flee.
Do let the PCs try their own wacky ideas. You may have it all set for them to resolve the situation in a certain way, but they will come up with some wacky plan that’s so crazy it just might work. Let ’em try, and have the NPCs just react naturally. One round of “What the-?!?” is not unusual.
Don’t let them push you around. While they might have good ideas, do not let them get into the habit of changing your mind. Rules questions can be debated after the game. During combat, any rules questions is answered “As GM, we play it through my way. We’ll discuss it after.” It just can’t bring the game to halt right then and there.
Do be willing to adapt when reasonable. After the game, do take the time to work out the best way to interpret the question, and make a ruling to use from then forward. Stopping a game to argue about a rule distinction kills it for everyone. The Players really do just want to play your game. Keep it Your Game, but be fair. Use your Iron Fist to keep things moving in the moment, but be ready to adapt the rules if it does indeed make sense.Make sure to mention this at the start of the game, especially if there are any Rules-Lawyers in the group