I think every author in the world through the course of time has been asked this question. I believe that there is a general curiosity among readers to know how the author came up with the amazing story that is on the page. I also think that readers believe there is some sort of magic well authors are able to dip into to draw out their ideas.
The honest—and not incredibly magical truth—is that story ideas come from everywhere. A lot of my stories (Life After the Undead, Finding Eden, and Humanity’s Hope) came from dreams. I had to flesh out the ideas and make them more plausible (so many weird things happen in my dreams that are hard to put on paper!), but the main idea came to me while I was asleep.
Other times, ideas are influenced by other stories and how the world (or I) react to them. Sometimes, something happens in the news that spurs an idea for a story. It’s possible to overhear a conversation in public that will plant the seed for a story.
The point is that there isn’t a magical well that authors go to for ideas. We don’t have special or exclusive access to ideas. However, it’s what an author does with those ideas that makes all the difference.
A good author can take a mundane idea that they get from the news or an overheard conversation and turn it into something magical. The author’s world building and character creation can make the reader believe that this idea was handed to them from above by a creature bathed in an ethereal glow—and that is the real magic.
Ask any author and they’ll tell you that they have dozens of stories that failed because they couldn’t make the idea work. It may have seemed great at the time, but as they put it on the page, it wasn’t as wonderful as they imagined.
Ideas can be found all around us at any given time. As authors, we have to be aware and open to receiving them and knowing what to do with them. We have to know which ideas are going to create a great story and which ones are going to fall flat—and that often comes with trial and error.