Elmore Leonard’s Rule No. 2. Avoid prologues: they can be annoying, especially a prologue following an introduction that comes after a foreword. But these are ordinarily found in non-fiction. A prologue in a novel is backstory, and you can drop it in anywhere you want. There is a prologue in John Steinbeck's Sweet Thursday, but it's OK because a character in the book makes the point of what my rules are all about. He says: "I like a lot of talk in a book and I don't like to have nobody tell me what the guy that's talking looks like. I want to figure out what he looks like from the way he talks."
Another thing I see a lot of in new writer’s work is a big fat world-building info dump of a prologue, particularly in high fantasy and science fiction.
There are probably a couple of reasons for it. Maybe the author is eager to establish the world of the story, and doesn’t want to wait to weave it in. Maybe they think the reader needs to be told up front. Maybe they don’t have the skill or training to do it.
Some agents and publishers will not consider anything with a prologue.
I don’t like to read anything that’s called a prologue. To me, it’s not part of the story, so I tune out. I have some kind of mental block toward them.
On the other hand, some authors think they’re ok. Donald Ray Pollock and a long one in The Devil All the Time, as does Nick Cave in And the Ass Saw the Angel. Neil Gaiman has one in The Ocean at the End of the Lane, and Stephen King has one in Doctor Sleep, although he calls it Prefatory Matters. And of course, Geroge R.R. Martin has one in Game of Thrones.
So, why not use one? If you use it the way the authors I’ve mentioned above used it, then go ahead. But most of the prologues I see don’t do what these do.
It’s true that prologue is backstory by definition. It should be something that happened outside the main story, necessarily before the main story starts. But the purpose of it is to establish an important fact so that when the story does start, the reader understands something about a character, or the situation he’s in.
Donald Ray Pollock does a great job of this in The Devil All the Time. He establishes the reasons for the mental and emotional state of Arvin’s father, and the nature of their relationship. It works because the writing is compelling, and it’s not an info dump. It’s told in the same way as the story.
Could it have been done differently? Of course. He could have called it Chapter One, or he could have woven it in as flashback, or reminiscence. It could have been a story he told a friend. The point is, though, it was not just pages and pages of detailed description about the world, or the history of the characters.
In Dune, Frank Herbert uses an interesting device to give just enough information about the world to provide the reader with the context of the story. There is one paragraph in which we are given the time and place, and a couple of pertinent facts. It’s presented as a quote from a book.
Many of the prologues I’ve seen from new writers go on for pages about the history of the world in which the story takes place, the life story of the characters and their relationship up to the point the story starts, and detailed physical descriptions of characters, and of the world in which they live.
None of that is necessary, and is not the function of a prologue. It’s not the start of the story.
My advice is to take out your prologue. If you insist on having one, take a hard look at it. If it’s just a bunch of information where the characters are not doing something, and there is no dialogue, i.e., it’s not itself a story, then take it out. Start with Chapter One and weave the information contained in the prologue into the story, to the extent that it matters.