A bit of a more high-level craft question today. How often do you like to bring in themes and elements outside the normal scope of the game?
By outside I mean like doing a "Horror" themed session in a game that is normally up beat and adventurous. Or bringing in the super natural into a game normally more gritty and less magical.
These outside elements can work as a wonderful change of pace, adding definition to the game and giving people a chance to see characters from different perspectives. At the same time though, there is a reason they're "outside" elements, and when people come to your game for Action and Adventure they may get annoyed if what they get is yet another Horror or Survival based adventure.
So how often do you mix things up? How much can you go to that well before you, or your players start to chafe?