So where do ideas come from? What makes one thought more special than another? How do authors take that one blip and turn it into a best seller?
Short answer: dunno.
Long answer: every author has a different way of going about the craft of generating ideas. There are ideas everywhere - and I should point out here and now that there is no copyright on ideas - from a sleeper's dreams to the overheard conversation to watching a bird take flight to the shopping mall, anything can generate the first kernel of a story.
That well of 'what if' never runs dry because we are surrounded by stimulus all the time.
It's true that writing is ten percent inspiration and ninety percent perspiration. Writing is hard work. Sitting for hours at a time, focusing on 'what happens next', is no easy task. The imagination is running off in particular directions - sometimes to a point you didn't expect, sometimes into a brick wall and sometimes it just works.
Writers are observers as much as participants in the world, we just view it a little bit differently. We look for motives, we look for action, for romance, for tragedy and redemption. We hear conversations from imaginary people, demands from villains who aren't there, confessions, plots, plans from thin air. We see fantastic landscapes, architecture, vehicles, weapons. We feel the pain of betrayal, the joy of discovery, the exhaustion of escape and passion of that first kiss; and all of it based in real life and all of it twisted and shaped and bent to our will into something new and lasting.
But no-one can write the story but the individual author. Writers can have the same idea, but the story will be different: C.S. Forester - Alexander Kent - David Weber; all stories based on Nelson, all different. Stephanie Meyer - Maggie Shayne - Charlaine Harris; all stories about vampires and all different. J.R.R Tolkien - Terry Brooks - Robert Jordan; all epic fantasy and all different.
In each case, the authors had similar ideas but wrote individual stories. And while cynics profess that there is 'nothing new under the sun', I propose they are wrong. Each idea is developed differently and into a new creation.
As for turning an idea into a best seller, well, it's timing, it's work and it's sheer, bloody-minded luck. It's up to the author.
Write something; write it now. Take the idea from the conversation you overheard in the post office, newsagent, bus, train or wherever and turn it into something lasting. No one has to see it, but it would be a shame for that idea to be lost. Who knows? That piece of work might be the next Big Thing.