A prologue is the first guest to arrive to the book party even before the guest of honor, the main story. It appears before the first chapter. Its content serves as an appetizer, whetting the readers appetite before the main course is served. It contains information which establishes context and offers background details that advance the narrative.
Think of the prologue as an account, usually chronicled by a character who holds some juicy secrets, the main character isn’t privy to. The events can take place during the same time frame as the story or years before.
The protagonist may spin a tale about what’s to come, however the writer must use their imagination and whip up details with a twist. Perhaps the main character is reading and responding to a newspaper article written about them, or they got their hands on their patient file and they’re reacting to the information. This must somehow foreshadow future incidents or hold a place card for upcoming plot.
If you decide to include a prologue, keep it short and don’t use it as a dumping ground for loads of info. Throw in a few nuggets about impending happenings, to spark the reader’s interest, and clue them in to what themes lie ahead.
When you’re banging out your prologue, drop in a similar vibe to that of your story and stick with a writing style in the same spirit. Just as your opening sentence and first chapter need to wow the reader, and capture their curiosity, so must your prologue. Make readers so intrigued by your prologue that they’re hungry for more and can’t wait to dig in to that first chapter. End your prologue with a vexing question that leaves the reader saying, “Wait . . ., what?” Or, better yet, stun them so they shout, “What fresh hell is this? I must read on.”
Prologues are written in 3rd person. Write the prologue in 3rd person, from a character's point of view other than the one used in the main story, or as suggested above. Even if your story is written in first person, write your prologue in 3rd person.