I've found myself thinking about world-building a lot while I read David and Leigh Eddings's The Elder Gods.
First, what I mean by "world-building" is all that goes into the back story of a novel—all the details that define the world and the rules of the world. In straight fiction, this means keeping the details of a novel to match what could happen in the real world. It gets a much more complex when you get into fantasy. For me, I had to answer questions like what are the types of evil creatures that can exist (demons) and which ones do I want to exclude (vampires)? Can plants have souls or is "soul-sight" a misnomer for what Madison can do? And—the thing I've worked the most on lately—what is the organization of the other-worldly company that Madison works for?
The most important thing about world-building, usually, is that it is seamless, subtle, and consistent—seen but not heard, so to speak; something you learn without realizing it.
In The Elder Gods, it is almost as if Eddings is doing the world-building right on the page, despite the fact that I know that he must have had everything predefined before finishing the novel. The majority of the novel revolves around the idea that a more-civilized nation of people is being called upon to help fight a battle alongside the gods' less-civilized people. In the process, the characters are learning of each other's worlds, what defines them, what works in them, what the characters are capable of and what is physically off-limits.
I'm now about two-thirds of the way through the novel, and the general world-building is virtually over, with all the characters now being integrated and on the same page, literally and figuratively. From here, all characters will advance through the plot together, and the world building will move, no doubt, to the backstage, woven subtly into the plot, once again unobtrusive to the reader.
The oddest thing about this up-front world-building is that, had I written this novel as my first draft, I most likely would have cut most of the first third of the book and started the story straight into the action, interweaving the world-building into the plot as subtly as possible. But it would have ruined the pacing, and most likely, the story itself. I still haven't decided if Eddings's choice to keep the book the way it is published is because of the genre (epic fantasy), where a certain amount of world-building on page is expected, or is the choice to keep the book as it is simply a decision of a master storyteller who trusts his work and knows what works for him and his fans. I'm leaning toward the master answer.