An "epistolary" story is told in the form of a document or series of documents: letters, journal entries, emails, transcripts, forum posts, personal web pages and what have you. Bram Stoker's Dracula, Mary Shelley's Frankenstein, and H. P. Lovecraft's The Call of Cthulhu are classic examples of epistolary fiction; internet examples include Ted the Caver and Dionaea House. Writing a story in this style has its benefits: it enhances the realism of the reading experience and can even make the reader feel like a participant. But it can also be difficult to pull off without coming off as hokey or unnatural. Here's a few tips to help you get it right.
Real-Time vs. After-the-Fact
A face-to-face conversation happens in real-time: when narrating a story orally to another person, it is perfectly natural for the speaker to stammer and stutter, or ask the listener to wait a moment while they deal with an interruption.
It is NOT natural when a person writing a document does the same thing, yet amateur authors who write epistolary fiction frequently overlook this. Think about the last time you were interrupted while writing an email: did you tell the recipient "Hang on, I have to deal with something?" Of course not! You weren't actually speaking with them! Letters, emails, and journal entries can only tell a story after-the-fact. They cannot be written AS the hero is engaged in the act of a firefight, or climbing down a rope, or fleeing from some terrible creature -- they are memoirs of the action that has already passed, however recently, which the narrator can only write during lulls in said action. It is vital to remember this if you want your narrative to be taken seriously at all.
So don't indicate an interruption by stating, "Hang on, there's someone at the door, brb" -- find another way to indicate it, or don't indicate at all.
Reading Between the Lines
Because documents can only tell a story after-the-fact, many details are more effectively hinted at, but never actually mentioned. We Don't Talk About Sarah, a story from Reddit's NoSleep community, does an excellent job of this.
The details the narrator actually gives us are, for the most part, mundane: a little girl gets a new baby sister, who then apparently dies and leaves her very sad; it's what those details IMPLY that makes the story so horrifying: the narrator's parents kidnapped someone else's child, then later murdered it to cover their tracks, and briefly considered murdering the narrator who may unknowingly be another kidnapping victim.
On the surface, the story is simplistic, kind of cute, a little sad, and not terribly interesting; but read between the lines and you find a tale of soul-shattering cruelty and depravity.
Establish the Narrator's Writing Skill
Suspension of disbelief is easily shattered when your narrator is a physicist who says "nucular" instead of "nuclear," or a seventh-grader who writes sophisticated prose with a broad vocabulary and flawless spelling. Establish early on how good a grasp the narrator has on the principles of writing (spelling, grammar, vocabulary, etc) and keep it believable and consistent.
However, make sure that it's still legible, too. Accurately reproducing the writing skill of a child can render the story unreadable, so you'll have to find a good balance between accuracy and readability.
Also consider how much detail your narrator is likely to go into. Are they more likely to summarize a chase through the labyrinthine belly of a submarine? or recount it in excruciating detail? Again, your narrator isn't omniscient: he/she is recording events after-the-fact as best he/she remembers them while likely still in an emotional state (or while reflecting in tranquility years afterward, depending).
Research
This is a must for ANY sort of writing -- research everything. Does the narrative record the progress of a military experiment that inevitably goes horribly wrong? Make sure the science of the experiment is based in fact. Make sure the proper procedures and methods are used. Make sure any official documents are written in the correct format.
At the end of the day, telling a good story IS more important than telling a factual one. Just beware that your facts aren't so fictional that they render your story cartoonish.
Why is the Story Being Told?
Epistolary style can't be used for the sake of it: it is imperative that the author know WHO the narrator is, and WHY the story is important enough to be written down. They might be writing a confession to redeem themselves; they might be trying to assemble their thoughts in a cohesive form before deciding on the next course of action; they might be sending someone a warning. But there has to be a REASON for the story to be in document form.
H. P. Lovecraft's At the Mountains of Madness tells the story of a geologist who, upon discovering and exploring the titular mountains, discovered unfathomable ancient horror that could easily be unleashed upon the world. WHY was his story told? It was a plea to a committee that was planning a second expedition to those titular mountains: in telling the story, he hoped to convince them to cancel the expedition and leave the mountains undisturbed. So not only do we get the story itself, but we're also left to imagine how the committee is likely to react (perhaps the true horror of this story is the horror of leaving such an important decision in the hands of a bureaucracy).
Which brings us to what postmodernists refer to as the "narratee." If the story is just sitting in an empty void somewhere, or the narrator is just babbling to him/herself, the story would never exist -- nobody is around to HEAR it.
As important as it is to know your narrator, it's almost equally important to know who the RECIPIENT of the story is within the story-world. Someone within that world is reading your narrator's words (the same applies to an oral narrative style).
Establishing the narratee's identity isn't necessarily required; the author only needs to know for him/herself, to aid the writing process. Knowing who the "urgent email" narrative is addressed to helps you shape the story: you know exactly how to address said recipient, what details will interest them, what details are relevant to the relationship between sender and receiver -- all of which helps to develop character and reinforce suspension of disbelief.
Though bear in mind that the email's intended recipient may not be the one reading it...if the email was never sent.
So to sum up, the best path to writing good epistolary fiction is to remember the following:
- Documents tell stories after-the-fact, not in real-time
- Important details are sometimes written "between the lines", implied rather than stated
- The narrator's writing skill and level of detail should be consistent and realistic, but legible enough for layman readers
- All your facts should be researched
- The story is being told for a reason by the narrator
- Someone (the narratee) is reading the story
Keep these concepts in mind when writing, and you should find yourself in possession of epistolary fiction that sells its readers on its absolute realism.