Going into the weekend my plan for the two games I run was simple. Friday would be a fairly laid back session, wrap up some loose ends, and then go smoothly into the end of season break with everyone having stuff to look forward to for next arc but no huge questions from the spring that weren't obvious hooks. Sunday would then be the drama and tension filled session with a chance of PC death and zaniness.
Fast forward to after game sunday, and while I did kill a PC this weekend they died on Friday not Sunday. L5R was tense and dramatic and someone died. Star Wars was mechanics heavy, but beyond that fairly light hearted as my PCs flexed the muscles on their 800+ XP characters and turned the tide. It's the kind of thing you get used to as a GM, but today I want to talk about how to make it work.
You Need To Roll With The Punches
Face it, even the best plans won't survive much more than five minutes of PC interaction. Things are going to go pear shaped, unless you want them to in which case they'll go avocado shaped, or just explode into weaponized glitter or something. The point is, no plan lasts. Your PCs will do something, and if they don't, the dice will. There are just too many RNGs involved in a table top session's decision making tree for anything to go predictably.
This is why one of the most important skills to have as a GM is the ability to improvise. The better you are at going "umm, ok..so here's what happens" the better job you'll do at keeping the fun in your game and not have it bogged down with drama breaks as the GM thinks through yet another thing - which, btw, just makes you look unprepared even if there was no way you could know the evil dwarf sorceror was going to summon a $*@9ing unicorn to help fight lizard men.
Don't Be Too Proud
In the wise words of Lord Vader "don't be too proud of this technological terror you've created. The power to destroy a planet is insignificant next to the power of the force." I don't know how to relate it all to gaming, but the point stands. Things are going to happen. Your plans are going off the rails. The script is going out the window. Don't be too proud.
What does that mean? It means don't try to force your plot. If you have 4 battle areas prepared for the "epic showdown" and all the PCs want to hop into a boat and create a 5th battle area you didn't expect? You should make that 5th area, no force the PCs into one of the 4 you prepared.
That monster you made to whoop on the PCs and be a big deal later on when they beat it? Yeah, that thing is going down first encounter. Expect it. Have other plans.
Celebrate Your Players
When a GM is doing his job right, the PCs will have every reason - from actions - to believe the GM is trying to kill their PCs. However, the GM will still be on the PCs side. This is hard to do, and some may disagree with me, but in general you should be rooting for your players (unless their characters are awful people? Then it may be hard) but playing the villains right.
That means if the villain has an ability it can use to kill a PC, that ability should be used. However, if that PC has a crazy plan - or somehow manages to win despite all the odds - you should celebrate that.
As odd or bad it can feel to be rolling like crap for the monsters/opposition, sometimes it is good. As baffling as it can be that a PC somehow survived for the third session in a row, that's awesome that the character is still there. Celebrate the players. Cheer on the PCs. Don't pull the punches, but don't think of it like you have to beat them. Ever.
Expect The Impossible
On Friday I had an L5R character die because he asked the spirit of a powerful mahotsukai for the way to kill someone while 'sending a message' trusting a different spirit to keep the blood mage honest. The spirit trusted to keep things kosher failed the roll to realize what was being done, and the PC summoned an Oni with a blood ritual. The PC then chose to go back and fight the Oni alone. Not because it was the right thing to do, but because otherwise their clan might not believe they'd actually done their job and killed the target.
They almost won the fight too. 5 points separated them and giving the Oni enough wound penalties that its attack would have missed. You can't plan for that. Any of it.
You have no way of knowing when your PCs will summon a devil to barter for a needed item. You don't know when the PCs will try to sign up with the bad guy and do whatever. You have no way of knowing when the thief is going to take an insult the wrong way and light half a city on fire. There's no way anyone can know these things...but they will happen.
Expect the impossible to happen. You won't know the form, but if you can at least know it's coming it won't surprise you as much. Then, maybe, you can react.