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.
- 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 David & Leigh Eddings did something similar for the Belgariad and Malloreon series, although theirs were more faux historical documents, with each book's prologue having a different tone and style and giving a different piece of history.
- 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.)