In a tabletop, the NPCs are all you. They do exactly what you want them to do, drop hints exactly when you want them to, and never do anything that surprises you, (unless you have multiple personalities, in which case, I want to game with you).
Obviously, with a LARP, this is not the case. Your NPCs are 5-20 different people, and they only know as much as you tell them. If you have experienced NPCs that you can trust, you can send them off to do complicated mods on their own. If you have a bunch of new kids, you have to watch them a little more closely.
My biggest piece of advice here is to trust your NPCs. If you try to micro-manage with your NPCs you are going to go crazy. Nothing is ever going to go the way you want it to, and the sooner you come to accept that, the more fun you will have. You have to be ready to adapt to surprises when running a LARP. There will be times where your NPCs will get a line wrong, or tell people the wrong thing, and you have to be ready to change your story to fit it.
But Red, what if my NPCs tell the players that a main character is evil, or dead, or just tell the PCs something wrong?
Simple! Have a more trustworthy NPC character come out and say it was all a lie! Or a mistake! Just because your NPCs have said something, does not mean it is cannon. Slips like this are easy to deal with.
But Red, my NPCs were supposed to die in a battle, but the players ran away!
Well, it looks like your PCs just gave you MORE to work with! Rather than worry about possibly running out of entertainment for the players, they have just given you an easy 2 hours. Simply attack them again! Or in a different way!
MY NPCs have blatantly ignored my instructions and did whatever they wanted!
Are the PCs having fun with it?
Go take a nap. Your NPCs are obviously doing a good job. Just because it is not what you planned doesn’t make it bad. Take a break. Frankly, I have never met a LARP GM who was not overly stressed. The nap should do you good.
But what if the PCs weren’t having fun?
I find that the players often forget something un-fun or boring if you just assault the town with something crazy. Or send them on an insane treasure hunt led by a goofy NPC. Remember, if you end big, they will remember your event fondly. Follow up something disastrous with an NPC called Billy the Bomber, or Dynamite Pete, and make the PCs dodge explosions for an hour or so. They will be fine.
You have to give your NPCs some leeway, and let them add their own ideas and character to the parts that they play. If you are too controlling, your NPCs will become bored or resentful, and that bleeds into how they interact with the players. Give them some ground to be inventive, and you won’t be disappointed.
The best mod ideas I have ever seen have come out of my NPCs sitting around, waiting for the next assignment. (Except bubble-gum elementals. I will never forgive you for that, Courtney.)
Also, remember, while mindless NPC villains can be fun (wolves, zombies, Halo Players) they can get dull after a while. These get dull for both the players and the NPCs. Give your NPCs, even your BRAND new ones, speaking roles.
Even if you don’t know if you can trust a new NPC, or how well they will perform, give them a part to go out and have fun with. Cast them as an annoying fairy, or a bumbling farmer. Let them be the hook for a mod that is not too important, or let them go out and gamble with your players. This makes them feel included, and it gives your players someone new and different to play with.
The more responsibility and freedom you are able to give your NPCs, the less likely you are to have a nervous breakdown mid event and run out of the camp with a trail of bodies behind you.