Let start with the basics. There are three things that will require improv skills: Story, Conflict, and Dialogue. Conflict and Dialogue will be covered in later posts.
StorySo you think you know how things are supposed to go. Unfortunately your players have decided that the most important person in the game must be the decrepit Hobgoblin on the outskirts of town who is trying to peddle his homemade snot-balls to anyone passing by. He was just a foot-note in your city description but your players think he is the Evil Mastermind of a yet undisclosed plot and they intend to spend the next hour or two getting to the bottom of his true purpose.
What do you do? Do you keep telling your players he is just a nobody and steer them back onto your plot rails or... do you go with it?
Here's the thing, if you go with it but don't let on that you're having to improvise everything that is happening, your players will think they have outsmarted you. They will feel a sense of accomplishment that they have bypassed all of your clues and found the real culprit. Just keep your mouth shut and smile, things are going your way. The Hobgoblin just got a promotion.
I like to add a big twist. Yes he is up to something, but it's not at all what the players think he is up to. Maybe he is a scout for an invasion force. Maybe his snot-balls are actually healing snot (taken orally) and the Hobgoblin is a prophet. Maybe he was hired by the real villain to keep an eye on who is coming and going to the town. That last option can be used to lead the heroes back onto your original plot if necessary.
Or you could provide them with proof of his innocence by making him the victim. Perhaps the real Big Bad Evil Guy needs something from the Hobgoblin Peddler and has sent some minions to rough him up.
The hard part here is pulling another story out of your brain pan. If you're used to playing things loose with the adventure you had in mind then you should be in good shape. If you have a very tightly woven adventure where things need to happen in a specific order, you are going to be frustrated and panicked (and neither is conducive to creativity).
The best thing you can do is just have fun. Make up something outrageous, and don't worry about the details too much. Flesh it out as you go. Answer only those questions that come up. It may sound terrifying but just trust yourself and let it go. Just make sure that it has a thread that ties it all together.
For instance, if my villain is a desert prince but I decide his servants are shark-men it won't make much sense for the two to work together UNLESS the desert prince has hatched a plan to flood his lands to make them more fertile (which will likely wipe out his neighbors ecosystems but what does he care). Newly submerged territory would be something the shark men would be very interested in. The water plot ties these two disparate groups together.
The GMs Secret Stalling Tactic!Let's say your players have completely blindsided you and you aren't ready to bounce back at them. You need a moment to collect your thoughts and regroup. Don't worry, take a break. If you don't want to tell them why (and why should you?) just call for a bio break. Then take a few deep breaths and regroup. I don't recommend taking a smoke break. I used to smoke and I'm glad I quit. Also, I have never had a moment to think while I smoked because some of my friends would join me or take the opportunity to ask me questions. The point is you need to be alone for a few seconds so go to the bathroom...alone...preferably. Just don't take too many of these breaks or your players may become suspicious...or worried.
Now just focus on...
What You NeedRemember, the object is to keep yourself loose so you can spring in any direction you need to go. You aren't going to flesh out an entirely new adventure in the bathroom, and nor should you since your players will just derail that as well. All you need is an Adventure Seed (just like the one's I have been tossing your way every Monday).
You just need a Villain, a Villainous Plan, and an idea about the consequences should he/she/it be allowed to succeed.
Letting the Players Write Your AdventureOkay, so we're back to that Hobgoblin. The players are convinced he's a bad guy. You've decided that he's actually peddling a narcotic while scouting the town for an invasion. Your player's don't know any of that yet.
Then one of your players says "He may be consorting with a demon. We should check that out. We'll follow him and use detect evil."
The Hobgoblin wasn't consorting with a demon a second ago, but that actually sounds cool to you so TAKE IT.
"As you follow him into the deeper forest he begins to radiate Evil," you say.
Player 1 - "Ha! I knew it."
Player 2- "Wait, why didn't he radiate evil before?"
Don't answer him. See if another player answers him first. This happens a lot at my table.
Player 1 - "Obviously there is something in the woods that keeps him from hiding his Evil."
That is how players will do all the work for you. Staying flexible. You have gone from the Hobgoblin being a flavor NPC, to a spy and drug peddler, to a Demon whose lair is deep in the forest. All you've had to do was change your adventure seed. After a while, your players will settle into the story and then you can start planning Conflicts.
Where Things Are Likely to Go WrongInformation Gathering
In this part, the players learn about what is going on, what is really going on, and who the villain is. For mystery games, this is 75% of the adventure. For your typical Pathfinder Society game, this is the first 20% of the adventure because you are handed a mission, told what is going on, and who the bad guy is.
This is also the most likely place for your pre-planned adventure to go off the rails. It's the place where your players are still looking for direction and focus and may go off on a different tangent. That is one of the reasons Pathfinder Society skips that by handing you most or all of the information the PCs need up front.
The heroes are unlikely to enter combat in this section so you won't really need any NPC's with combat stats at this point. Honestly all you need is your adventure seed and a rough idea of where it is going to go. If you have proactive players like in the above example, they may end up creating this entire section for you.
At the end of this part, your heroes will have their story and now they just need to assault the villain's base and take him down or catch him in the middle of his villainous plot and stop him.
So Why Do I Like Improv So Much?Because I'm not just reading an adventure to my friends and waiting for them to get the right response. I am thinking, planning, restrategizing, and I don't know what's happening next. In short, I may be running things but I am also playing. I think too many GMs don't actually play when they run, which is why I think so few people want to GM.