For those of you crafting new worlds, or just modifying our own, I want to talk about believability. Google is telling me that isn't a word, but I got a Linguistics degree instead of an English one. Anything you need to be a word, that you can make understandable, is a word. If it carries meaning and works within your grammar? It's a word.
Back to believability. Your new world, how does it work? I'm not really talking about your mystic cosmoses and Gods -- you can do whatever you want with that. But the basics of your world, how people get their food, their wood, their cloth. Where does all that come from?
If you've written a story about a remote location with a local hero, does everything in their story match what a place like that could really have? Look at the resources in your world. See what's available to the people where they are. Then compare that to what you have in your tale. Maybe you've discovered that in the place you've put your story, sheep do not exist. What are the clothes made out of? How is that resource farmed?
Do you have a huge city surrounded by forest? Where do they get all their wood? Do your people use metal? If so, where are the mines? If there aren't any, where do they get it from?
These details won't make or break a plot, but they will enhance your storytelling. Answering small questions about the origin and value of normal resources from our world that you have transplanted into this new one, or new resources you yourself have created, will give you new stories all on their own. Farmers in the region, miners, workers of all kinds will fill your marketplaces and shops and homes. Answering questions about scarcity and resource management will create jobs, and jobs create NPCs, and NPCs make stories great. You are following the protagonist, of course, but often you are asked to be the protagonist, which means that your story revolves around the people around you.
Never neglect your NPCs. I'm going to quote it again, because it makes me happy.
Everyone is their own main character.
That said: Don't focus on these other people you create. Know who they are, and how they talk, what they care about. Doing this will give you believable dialogue, and fill voids in your world's maps that you didn't even know were there. But you are writing a story with a protagonist, an the story should be focused on them. They're where all the action's at. But these other players are important in their own right. Marion Zimmer Bradley's Mists of Avalon is an amazing example of this kind of detailed worldbuilding. Everyone is real, no matter who they are or how little you see of them. And they don't all just exist to die. (Hint hint GRR Martin cough cough)
Er. Yes. Now this next part might be a little too specific for some stories, but it's something that bothers me about GRR Martin's Song of Ice and Fire series, and some others. That's right. I'm going to whine to you about...
Not for long, I assure you, but just think about why you've chosen to base your world off of ours so readily. It's easy, absolutely. If that is your reason, that is fine. But I believe in crafting unique worlds. It takes more effort to build your own personal De Rerum Naturis, (or Monster Manual), it will make your story more vibrant and alive. Miyazaki's Nausicaa is an astounding example of this, but the plants in Nausicaa are basically the point.
You say okay, but if it doesn't have to do with the plot, why should I bother?
Let me ask you this. Do you know anyone who saw the movie Avatar*? That movie was a carbon copy of Fern Gulley with more different plants and dinosaurs instead of fairies. No one went to that movie for the plot!
And before you call me out on it, yes, Unobtanium is a terrible name for a mystery resource. That is just terrible.
But you didn't go to that movie for the dialogue, either. You went for the monsters, and the plants in the vibrant 3D.
When you're reading a book, you are crafting a mental movie. When you're writing a book, why not make it a visual Avatar, with a real plot? Not just Pocahontas (or Fern Gulley) but with... this?
I dare you to go out and create something new.