All of my GMing rules posted so far are collected here.
GMing Rule #3. Don’t invalidate the players’ efforts.
PCs and Protagonists
In every story, the hero struggles. Man versus man, man versus nature, man versus himself. This is as true in wars between demigods as it is in an English country veterinarian’s memoirs.
In an RPG, though, that protagonist is a player character – with a player behind them.
That player is a real person, and that real person is investing real time in the game. It behooves a good GM not to waste that time.
One particularly conspicuous way of wasting that time is for the GM to allow the players to spend time and energy struggling toward a goal, and then invalidating that struggle.
There are two main forms I see invalidation taking. In the first case, the GM inflicts failure on the players, no matter what ingenuity or determination the players bring to bear.
This happens when the GM sets up a plot that can only advance if the players fail – a plot that halts if the players ever succeed. A Lost or a Star Trek: Voyager, or a Gilligan’s Island, where every adventure is an attempt to get home, but where the simple math of the premise demands that all of these attempts fail.
I’ve seen this structure in RPG sourcebooks. I have a sourcebook ready to hand that outlines a campaign about the approach and confrontation of a mighty and terrible evil. Every adventure in this campaign is about some antagonist taking steps to advance the evil’s approach, and every one of these adventures requires the players to fail to stop these antagonists – or the campaign is over.
At every step, no matter how cunning or steadfast the players, they are guaranteed to fail. Their efforts are wasted.
The second case is where the GM allows the players to succeed, or to come close to succeeding, and then reveals the players’ cause to have been a hateful one.
There’s a video game I could name, wherein the player’s objective is to rescue the player character’s girlfriend from a distant castle. At the end of the game, it’s revealed that the girlfriend is the PC’s ex, and she’s been actively trying to get away from him.
Again, the players’ efforts are wasted – wasted and soured. You’ve just taken all of the time and energy the player put into that goal, and made it retroactively un-fun.
Frustration vs. Invalidation
I’m not saying don’t make things difficult for the players.
Knock them into pits, so they can fight their way back out! Set them up against unbeatable odds, and then see how they beat them anyway. Put the goalposts on the damn moon, and hand the players half a roll of duct tape and an empty egg-carton to get them started on a rocket.
Make them struggle.
But don’t waste that struggle.