There are some minor Game of Throne spoilers in this post. If you’ve read anything beyond a few chapters / the first episode of season one, you should be good.
I think the best prologues are never required. Instead I find them to be the equivalent of a shady guy standing in front of a store.
Some people might ignore him and move on, while others ask him what he’s up to. No matter their motivations for investigation all that, are wordlessly handed a key.
The story, the store in this analogy, has tiny keyholes weaved all through the products. For the people who questioned the shady guy, they are rewarded with a mystery. Curiosity may drive them to put their key into these keyholes as the early chapters play out.
The ‘key’ needs to be digestible morsel. It needs to be a notion that doesn’t detract from the early story but instead something that gives the reader short pause. They key doesn’t fit? They move on.
Making the reader wait too long may only lead them to toss they key aside and move on. Worse, they might walk away from the story and never return. Do it too early and the mystery is cheapened, your reader won’t appreciate the mystery. The “Why wasn’t this chapter 1?” mindset sets in and they may see it as a cheap gimmick.
I think the best set up is when the doesn’t notice the keyholes unless they pick up the key. To the uninformed, the keyholes may come across as random details with no bearing. When I find the correct keyhole and get that satisfying click I rejoice.
The reader’s diligence should be rewarded with a revelation (hopefully an interesting one). Back-story isn’t a good enough key.
Using Game of Thrones as an example. The prologue involves a man from the Night’s Watch abandoning this duty to run from a proverbial ice monster. When you see this guy again, he’s being executed by Ned Stark for deserting his post.
Someone that didn’t read the prologue would think: “Wow, this guy abandoned his duty, he has it coming.” While someone that read the prologue might empathize with him, knowing he never stood a chance had he held his ground. Consider this when making a ‘prologue’.
Is there a mystery justifying your reader holding onto that ‘key’? Or is it just thrown away at first opportunity?
That’s the gamble.