Sanguvixen
J.K Rowling put good thought into characterization which made up for how unbelievable the Harry Potter world is.
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However if your characters are good enough, the world doesn't have to make sense.
1: I know several people that would love to disagree with you on that point, myself included when it comes to the later books.
2: Uh, no. Everything has to have a balance. Characters, plot, writing style and world all have to be relatively balanced in order for a story to be enjoyable. Brian Jacques and J.K. Rowling got away with as much as they did because the world was relatively balanced with the plot and characters, and their writing styles are unique and easy to read.
A severe deficiency in any one of those four things is, quite frankly, the sign of a bad author. I wouldn't want to read a book where it's clearly stated in one chapter that horses are the fastest and most efficient means of travel in that world, and a few chapters later see the characters on a train. It shows absolutely no thought or planning in the world, and is a huge turn-off for many people.
Aside from that, it's also a way to fit in Mary-Sues or Gary-Stus. If there's no preplanning in what is possible and what isn't in a specific world, who's to say that something
can't happen in the story?
I put a lot of thought into my worlds. Admittedly, I'm horrid at making maps, but I know how the magic system works, a general layout of Direnla city, and can even tell you about the culture and government systems. All this allows for consistency, so, for example, I know that when I write about a certain spell, I can make sure that the results are the same every time (unless it's poorly made and backfires, in which case I can also say how it will react).
I'm also not saying that people should do the amount of planning I or other people in The Creative Process have done in their own stories, but at least some thought has to be put in to avoid inconsistencies. If you find inconsistencies, do away with them and put at least some thought into how it could have been avoided.
Another example: Even if you don't make detailed maps, maybe make a rough diagram saying that Country X is northwest of Country Y and that Country Z is to the east of Country X and northeast of Country Y while being directly north of Country A. This way, you can say relative directions every time your characters are discussing the appropriate countries.