Insane is very hard to play "right". Its easy to take care of the bit about sometimes being nonsensical, but more often than not, the acts of the insane aren't nonsensical. They're entirely justified. In their own minds, that is. While its no doubt fun to play the vampire that slaps his or her prince with a fish, that same vampire, presuming he or she is insane, is more likely to kill someone for wearing a blue shirt than to slap someone with a fish.

The best playing of insane results in true chaos for everyone. And of course the level of insanity varies greatly. Some of the insane have no self control at all. Others are simply a bit... off-kilter.

we've found it interesting to run the worst character concepts we get thrown at us as a GM through the universal Mary Sue litmus test. Its unfortunately rather common for them to fail it. Miserably.