Piratecat
Sesquipedalian
Right. This is the best advice you'll ever get on DMing.Try to say "Yes" when players want something. If it makes your DM senses uncomfortable, go for "Yes, and..." so that you take their idea, but weaken it or apply something else to it. If it's too outragous, try "No, but..."
It's not easy at first, but the results are wonderful. And if there's ever any sort of rules conflict, my default is "what's going to be the most fun for the players?" Mind you, that doesn't mean it necessarily gets decided in a way that is easy for the PCs, but I always default to whatever is coolest.
Regarding your NPC? Try using less powerful NPCs that the heroes could beat up if they wanted to. Make them important politically, or just nice guys, instead of physically powerful. Then always think about the consequences for what the heroes do. It's possible that an exciting story starts as soon as they ignore your planned adventure and kill the guy giving it to them.
An example: Okay, your PCs have just ganked the person giving them their mission. You're panicked and angry. What to do?
Well, you could throw guardsmen at them to try and arrest them. That doesn't usually end well. A niftier solution might to have them immediately designated heroes. He turned out to be very famous, very evil, and very wanted by the law in a nearby country. The PCs get a parade and public celebration. Then all this guy's former colleagues show up seeking retribution... and what if it wasn't really him? Why is he wanted? Are the heroes being set up, and if so, why?
Figure out what your players expect, then turn it sideways. Keep them on their toes. They'll love you for it.[/spoiler]