Genre makes a big difference when you are deciding what sort of challenges to put in front of your players. This is especially true in certain genres.

If you are playing Call of Cthulu then at some point there should be an encounter with knowledge that man should not know. If you are playing a western game, there’s going to be a gun fight. If you are playing Star Trek there’s going to be an encounter with Klingons.

Combining genres can also be a way to help you build an adventure. You may be playing standard D&D but if you do a murder mystery adventure, you have a ready-made structure to build your encounters on. You need a body, you need clues, you need a murderer. Be careful not to railroad and leave plenty of clues laying around that will lead the PC’s to the murderer. A lot of times, you can straight rip ideas out of books and movies without anybody noticing when you mix match genres.

There you go. Try putting your fantasy chocolate in the serial killer peanut butter and see what you can come up with.