But my book is set in the 1920's. The real 1920's. No need for world building, right?
One of the most important parts of sci fi and fantasy world building is deciding on "the rules of the world." If you decide that wizards and witches generate their power by chowing down on the legs of spiders dipped in tabasco sauce... well, weird, but ok. It's a rule. You establish it early, and you're expected to stick to it for the remainder of the story.
In more reality-based genres, a lot of the technical rules have already been established (the sun rises in the east and sets in the west). But I still have to answer some metaphysical questions about the way my world operates. Such as:
- Do bad things happen to good people?
- Does true love conquer all?
- Does everyone get their just desserts?
- What are the roles of various groups?
- What is the role of accident and coincidence?
- What kind of humor exists in my world?
No matter how factual the setting, I have a lot of leeway to play with all of these things. And I have an obligation to keep all of them consistent. Otherwise, I'll get the same reaction as the fantasy writer who suddenly has her wizard draw power from eating butterfly legs dipped in ketchup: