In the Wall Street Journal Speakeasy section Michelle Kung tells how Veronica Roth came to write her book, "Divergent," which is in pre-production as a film with Summit Entertainment:
Roth...first came up with the idea for the story while driving between Carleton College, her first school, and Northwestern, where she transferred. She imagined someone jumping off a building, not for any self-destructive reasons, and that became a germ of a idea for “Divergent,” which does feature a large Pit within the world of the Dauntless faction. Finding the concept fascinating, she wrote 30 pages before setting her story aside, thinking it silly. Luckily, she picked it back up four years later and a trilogy was born.
Her books are part of a trend toward dystopian fiction for Young Adults. At the risk of making you hate her, I'll tell you that Roth is only one year out of college, she got an agent right away, they got an offer four days after the manuscript was submitted and a film deal a few months before the book came out.
My main point in this post, though, is to remind you not to throw away any ideas. In this case, a vague thought grew into 30 pages...and then nothing happened for four years. And then a lot happened.
(find lots of practical guidance in "Your Writing Coach," published by Nicholas Brealey and available from Amazon and other online and offline book sellers.)