One of the standard symptoms of insanity as it is portrayed in television and film media is paranoia accompanied by graphomania, usually expressed by writing on walls, tables, body parts, etc. It can be the same phrase written dozens of times, or an elaborate theory detailing a supposed conspiracy. Confessions, mathematical equations, rants, and screeds are also popular. These can be supplemented with pictures, newspaper articles, official documents, etc. Usually, a character will find some room in their environment upon which to fully express the terms of their obsession. And don't expect these characters to be deterred by the fact that they Couldn't Find a Pen; they'll write with their own blood if they have to.