I always had the story plotted out from the beginning. I sold the series as a trilogy. That means I had to submit a detailed outline to the publisher of each book in the series. Your publisher wants to know not just that you know how to start a story but also that you know how to end one, and that nothing too crazy happens. So I had the story plotted out, because it was required. I also always knew it would be a trilogy. It is structured on the hero’s journey to the Underworld — the theme of the first book is descent, thus each epigraph makes reference to descent (“The Descent Beckons”, etc.). The theme of the second is hell or the underworld, and all the epigraphs make reference to hell or the underworld (“the Gates of Hell”). The third book’s theme is ascent or heaven, and all the epigraphs make reference to ascent or heaven: “The Road to Heaven.” As for the second trilogy, the themes are temptation (“Fallen Angels”); the fall (“Lost Souls”); and redemption (“Heavenly Fire.”). It took me about two years to write the first book and develop the world: since then it’s taken me about a year per book to write the others.