Last time I blogged about prologues, I did so under the title "Why you probably shouldn't, why maybe you should", and I do stand by that. A lot of the prologues I see are trying to do something which would be better done another way. At the worst, they're trying to solve a problem with how the rest of the book works, but just cause a worse problem. At best they're surplus to requirements, and weaken a beginning that would otherwise work well. So I'd suggest that your first reaction to wanting a prologue is to see if there's a better way to do what the prologue's trying to do.
But then I dropped in on Jane Wenham-Jones's pilot for a new YouTube series, based on her hilarious and also excellent Wannabe A Writer books. In Episode Two, the lovely - and seriously bestselling - Katie Fforde talks about how the reader "locks on to" the first character they experience, like a baby bird imprinting. (The other episodes are well worth watching, too.) You need to make sure that that's the right character, says Katie, and that we know immediately what her situation and her problem is.
Katie's absolutely right, of course. The opening of a novel establishes the most important character or two and what their essential problem is: what they're trying to find or get, and what chasm is yawning before them if they fail. This is the person whose fate we must care passionately about. This is the character who has so much at stake that the opening can implicitly promise, as Andrew Stanton puts it, that this story is going to be worth the reader's while. Katie's example is, you could say, the women's fiction/romance equivalent of having a body on page one in a crime novel: in those, what you want the reader to imprint on is the problem that a dead body automatically poses, and the narrative is the story of how the problem is solved.
Mind you, at the more literary end of things, (see here for my exploration of the difference) readers are willing to stay for a while in a more uncertain, ambivalent, ambiguous narrative, where it's not quite clear what we're hoping for, what we're fearing, and whether it will all be be worth our while. And if that's the kind of thing you hope to write, boy will your prose, ideas, voice and general literary wonderfulness have to be staggeringly brilliant, to make up for denying the reader that clarity and immediate connection, and that clear forward drive.
Meanwhile, the rest of the reading world needs to know who and what they're following, and why. So what I'm wondering is whether this also explains the proliferation of prologues these days: it's a way of starting obliquely, as more literary fiction may, but without risking the reader imprinting on the wrong character and situation. The fact that it's a prologue warns us off; it says, "You need to know this, but watch out, this isn't the real story." Maybe that's a good reason for having a prologue.
But still, I'd say, you the writer need to be careful too. There are two halves to what the existence of a prologue is telling the reader. How you write it may well say very clearly and compellingly, "You need to know this", and really make the reader want to know it. But are you sure that the other half - "this isn't the real story" - is a good enough way to start your story? Is that really the best way to make the promise that Andrew Stanton talks about? What in your prologue is telling the reader that the following 300 pages are going to be worth our time?