To sum up: Have your villain make mistakes. This will surprise your players and make them double-think the strategies for their game. I’ve had this done to me time and time again as a player. I’ve always thought, “Why is the Bad Guy doing that?” and I’ve come up with some hare-brained plans because of it. Sometimes the GM makes an honest mistake in his strategy. Sometimes the GM is just throwing us a curve ball and trying to make things easier on us… and we just ended up taking the hard route.
Know what your PCs are capable of before you throw things at them. As a GM, you don’t want the PCs to have automatic successes, but you want things to be possible. As a an example, I had a GURPS GM throw a spirit at us that could only be contained for a few seconds at a time, but required a complex exorcism spell to dispel. Unfortunately, the exorcism spell was too high level for our power level, and we couldn’t get rid of the nasty spirit. We did have a lower level spell at our hands, but it took ten minutes to cast. The end result was that we had to run away, recruit an NPC to do the dirty work and come back later. Fortunately for us, the GM was flexible and allowed us to find said NPC even though he didn’t think we would need them at the start of the adventure.
I’m currently reworking my RPG system in my head. I’ll be putting things down on paper soon, and this post caught my eye. I considered it, briefly, as a core mechanic for the RPG system, but I dismissed it as being too whimsical for the nature of my game. However, it’s still a good lesson to take into consideration. If a combat is dragging on and it’s clear that one side (probably the PCs) is going to win, then allow them to get in a a few “good hits” and call it quits by saying the Bad Guy goes down.
Here’s another GURPS story for you. I was running a GURPS game in which a player had built a thief-type character. My concepts of what skills a thief should have and what skills the player actually picked were drastically different. I made some suggestions and even built out “Joe Thief” to show to the player what I thought a modern day thief should be capable of. My suggestions were ignored, and I didn’t realize this. As the game drew on, I realized the thief would not be able to perform any thiefly functions in the manner I envisioned. I had to adjust how the game was run, and the game suffered. My point, which is not related much to the horror genre, is that the GM should know what the characters are capable of, especially in an open-minded game system like GURPS. When designing your campaign, make sure the players are acutely aware of what you will expect out of them.
This post typically relates to the higher power characters where they have strongholds, investments, businesses and the like. Some systems, such as Burning Wheel, allow for PCs to start with substantial holdings. Once the PCs acquire such a thing, then they should become integral to the plot of the story. They are there for the GM to leverage for and against the PCs. If a character has a shipping company or an import/export business, then cut off some of their trade routes and inspire the group to venture forth to figure out what happened to the formerly abundant supply of a certain set of goods. It can make for a fun time.