One of the key problems with epistolary literature (or really most anything told in the first person) is the simple fact that eventually what you're reading has to be written down. What you're reading is not only an account of an occurrence, but its an account given ex post facto. If you want a good example of the problem that this paradigm creates, read The Pit and the Pendulum by Poe. Tension in horror stories comes from us identifying with the main character, or otherwise relating to or feeling sorry for them, and being ultimately uncertain as to what will happen to them. So, unless the writer does that lame cliche of cutting off the narrator in mid-sentence (as even I've done), or leaving things on a cliffhanger promising a future account that will never be (something I've also done), the story is spoiled because you know they survive, at least intact enough to dictate the preceding memoir.
Epistolary Rough by *533497 on deviantART
