Fiction suits are a concept introduced by Grant Morrison in the comic book The Invisibles to describe a method of using a fictional character as an avatar to enter a fictional story and alter it. This practice is considered a form of narrative magic.
The magician creates a character and wears it as a fiction suit in a story they are either writing or participating in rewriting. This is done simply by writing one's self into the story. When written with intent, narrative magic will often have an effect on the life of the caster in the form of strange parallels with the story being written.
One example of this was in the comic The Invisibles when Grant Morrison wrote himself into the story as King Mob AKA Gideon Stargrave AKA Kirk Morrison. Famously, after he wrote a torture scene of King Mob he experienced physical illness, great suffering, and nearly death.