If there’s one thing that can kill the mood or enthusiasm of a player attempting to perform an action in character, it would be being told by the GM that he can’t attempt it. Such things snap the suspension of belief, much in the same way that videogames can break a player’s suspension of disbelief when their highly athletic avatar can’t jump over a two-foot garden fence.

On the other hand, players can (and will) be audacious. They’ll try all sorts of things that may or may not be in line with what you had in mind when you crafted a scene. For example, in a game where they play supers, a player may just decide to fly to a country ruled by a despot and “backhand him so hard that his head spins around thrice before falling dead to the ground.” in an admittedly ill-considered attempt to bring freedom to the masses.

At this point, most GMs will tell the player, “No, you can’t do that.” But if the game is about power and responsibility, then doing something like this might be a springboard for all sorts of interesting complications. If I was running this game, several things could happen in reaction to that act:

The despotic country’s remnant government rallies together and makes this an international incident and a reason to declare war.

The despotic country’s government dissolves in fear, and anarchy erupts as the masses riot and / or panic.

Several other supers from other countries decide that what happened was a good thing, and start assassinating world leaders whose views are opposed to their own.

Each of these outcomes represent the consequences of his act, and ones that his character will have to then deal with, whether by escalating further (always a good source of plot hooks) or trying to fix the situation (another good source.)

The key here is to permit the action itself, but to modify the outcome, by either the And or But approach. These approaches work by qualifying the nature of the attempt or the success of the action taken. The important part is that it makes the nature of the attempt interesting, and gives an opportunity to further the plot.

I don’t think I’d have to state this out loud, but for the sake of completion, the Yes, But and Yes, And methods aren’t for devaluing the efforts of the players either. So please, let’s try to stay away from the “Yes, but it doesn’t work.” interpretation.

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That said, I think it also bears saying that one of the most powerful tools a GM has is communication. A lot of situations that have made fledgeling GMs quit out of frustration could be avoided with a little talk with the players. Barring truly annoying types, most players are willing to scale back, or revise their actions if you tell them that they’re straying from the type of game you’d like to run.