science fiction - When do I explain my created world scenario in a prologue vs. letting it unfold in the story?

Let's say I'm creating a unique world for my book. New planet, maybe new species, complex society with complex rules, history, government, and so on. Some of these details are absolutely necessary to understand the plot of the story.

There are two ways to give this information to the reader.

1. Prologue of some kind: Anne McCaffrey had this in the beginning of her original dragon trilogy, explaining how humans came to Pern, what dragons were in this context, what Thread was (both in scientific reality and what it meant to the Pernese), and how things had changed and developed to leave the society as it was when the story opened. David & Leigh Eddings did something similar for the Belgariad and Malloreon series, although theirs were more faux historical documents, with each book's prologue having a different tone and style and giving a different piece of history.


2. Jump in and learn along the way: Mercedes Lackey's Arrows of the Queen trilogy just starts, and we learn about the society and Companions as the same time as the 13-year-old narrator.

My question is: Where is the tipping point between explaining the world as the narrator is introduced and the story gets underway, and setting the scene before the story starts so that you're not infodumping and having characters say ridiculously obvious things for the reader's benefit?

At what point is a world so complex that you kind of have to explain some of it before getting underway? Is it always "lazy" to have this kind of prologue?

(I'm not talking about a recap prologue, by the way — sometimes you need a refresher, and I'd honestly rather have that in the prologue than have characters waste time thinking about what happened in the previous book just to remind the reader.)

Answer

If you can do it in the story, and the story will not lose on it, do it. If this would hurt the story, do it in prologue.

There are a few reasonable tipping points:

• BORING. If the elements of the world would not add to the story. It would be lengthy and tedious. Do a quick info dump and be done with it as painlessly as possible.


• No room for good cabbagehead. It's a team of experts, or a pair of elder gods. There is simply no room for a rookie or apprentice to learn along the way, no good excuse to deliver the lectures. Or opposite - the characters are all cabbageheads and there's nobody competent to explain what is going on, but the reader needs to know the essence. Stories too alien to introduce a reasonable cabbagehead will fall under this too.


• No TIME for cabbagehead. The story starts with action depending on the setting too much that the reader would be completely lost without some kind of intro.


• Perfect room for a prologue. Opposite of the necessities above. You have a great, bite-sized piece of history that is completely disconnected from the main tale, but introduces the concepts and simultaneously captivates the reader.


• Optional. You're writing a sequel/expansion which ties in with backstory of the original. These who read the original would find recaps in the main story tedious. These who didn't, would feel lost. So, "in last episode..."


• Hanging some chekov guns. You have no good opportunity to do that from the main story, but there are things you simply must foreshadow early on.

This list is not complete, by any means, nor ultimate - e.g. quite often you can do an interesting and captivating recap of past events in a sequel without need for an intro.