Prologues are always a hot topic around writers' water coolers (including virtual water coolers). Today's post was sparked by my seeing Curtis Brown's Sarah LaPolla (@sarahlapolla on Twitter) tweet this about prologues: "Whatever you think you need it for, you don't."
I've certainly seen books where I thought the prologue was totally unnecessary. Or even thought that it hurt the book by revealing the solution to a mystery that the main characters spent hundreds of pages solving, undercutting the suspense.
Prologues are also risky because they are the reader's first introduction to the work. The reader gets involved in a world and a set of characters, and then--boom, that world and those characters disappear, and we start all over in Chapter 1 with another world and new characters. Even if the world and characters overlap, it's disconcerting, and it's a lot of mental work for the reader to build two settings in such a short time. It also can keep the reader from really committing to the work--we know this is just a prologue, and these people are going to disappear in a couple of pages.
I'm not anti-prologue. (When it comes to writing, I'm rarely an absolutist!) I think prologues work best when they foreshadow rather than give an outright solution; when they set up a problem that the main characters will have to solve, and we're not sure how that solution will play out.
To test whether a prologue is needed, try this: Cut out the prologue and give the manuscript to a trusted critiquer who hasn't seen the story before and doesn't know the plot ahead of time. See if the critique produces a criticism that there's something missing, or confusing about the backstory, or any other comment that would point to the necessity of the prologue. If it doesn't, keep the prologue out. However, if there's a comment that only says the beginning is slow, that doesn't necessarily call for a prologue; it could call for a strengthening of Chapter 1 instead.