Worldbuilding is one of those basic skills that’s important for all writers, but vital for those of us who write in totally imaginary science fictional or fantasy worlds. There are two basic approaches, the soap bubble and the iceberg.
For the iceberg worldbuilders, there’s a whole lot of information underlying the stuff that actually gets into the story. What the reader sees is the 10% of the iceberg that sticks out of the water, but the writer has come up with a ton of supplementary supporting detail – politics, maps, languages, music, clothing design, law, culture and customs, and on and on. All this gives the imaginary world a tremendous solidity and consistency, because the writer has all this stuff to draw on…and the writers who do this usually spend an enormous amount of time researching and doing their best to make sure that everything ties together.
One such writer I know took advantage of living near a university to get input on his design. He started with the astronomy department, where he found an interested grad student who helped out with designing the star system, other planets and moons, and the composition of the planet’s crust. Then he went to the geology department, where he figured out the tectonic plates and mineral deposits; geography for mountains, lakes and rivers; agriculture for crops, and so on. As I recall, he ran aground in the climatology department – they were happy to point out basic climate zones, but when he asked about weather, they said “We don’t know enough; just make it up.”
Soap-bubble worldbuilders take a different approach. They’re all about illusion – they invent broad swath of interesting detail that’s shiny, hangs together, and implies a lot of history and culture and so on, but which has no more substance behind it than the air that fills a soap bubble. Everything is consistent and plausible on the surface, but surface is all that’s really there.
The advantage of the soap-bubble method is that there’s lots of room to make up useful background whenever the story happens to need it. The disadvantage is that the writer has to fit any new information in around whatever has already been established, or risk the reader losing their suspension of disbelief.
Note that soap-bubble worldbuilders are not necessarily just making things up as they go along. All the ones I know invent quite a lot of their settings before they ever start writing; they just aren’t going into nearly as much depth as the iceberg worldbuilders. What they all seem to have in common is a strong sense of what their setting is like. They will make up a really cool detail and then sigh and say “Yes, it’s cool, but it doesn’t belong in this world” even if there is nothing specific in their worldbuilding-to-date that would make that detail not-work. Sometimes, writers who absolutely cannot do any pre-planning in regard to plot or characters can quite happily do extensive worldbuilding in advance of sitting down to write.
When soap-bubble worldbuilders write long, multi-book series, either their worldbuilding starts to break down, or they become inadvertent iceberg worldbuilders as they accumulate more and more background that doesn’t actually need to be put in the next book. It’s very difficult to keep track of all the random details one has to make up for even one novel, even if one already has a lot of underwater background; it gets harder and harder as a series progresses. It’s easy to overload the soap bubble – to give just one or two bits of information that don’t quite fit with what’s already been said, so that the whole structure collapses. It’s the equivalent of blowing a little too much air into the bubble and popping it.
Iceberg worldbuilders have an easier time going on for multiple books without the worldbuilding collapsing, though if there was an unnoticed flaw in the initial underlying structure, it’s likely to be stressed under the weight of carrying more and more novels, until it either melts down or fractures. It is extremely difficult, in my experience, to come up with a fix for a worldbuilding mistake that will work retroactively, though it can sometimes be done.
The good news is that anyone who has a five or seven or ten-book series almost certainly has enough rabid fans to keep things going for quite a while longer, even if the worldbuilding is starting to show signs of problems. The bad news is that a writer whose worldbuilding is starting to break down is likely to be really bothered by it, to the point of having difficulty continuing (unless the writer really doesn’t care about consistency and believability at that level, in which case the breakdown probably occurs sooner rather than later because the writer didn’t put any effort into worldbuilding in the first place).
Whatever method one prefers, it’s well worth putting time into worldbuilding. Holes in the setting/world often translate into holes in the plot, and if the background is recognizable (i.e., real or based closely on somewhere/somewhen real), holes in the worldbuilding frequently mean that anyone familiar with that time/place will reject the story as implausible or unrealistic even if the story is a complete fantasy. Maintaining the reader’s belief in the story is important to every kind of fiction; consistent and believable backgrounds are a key ingredient in doing that.