I can't really speak for any author besides myself, so when I answer these questions that people pose to me, I am simply giving you MY answer. Every writer's answer may be quite different. But since I published THE TESTIMONIUM last year, several people have asked me this question, so I thought I would repeat it here and give you my answer.
NOTE: This entry contains a number of spoilers regarding my novel, THE TESTIMONIUM. If you have not read my book yet, please go immediately and purchase a copy, then finish reading it before returning to this entry. You will be glad you did.
(Also, let me know if the cheap marketing ploy above actually works!)
The question is this: When you begin a story, do you know how it will end? And how long it will be?
The answer is pretty simple: NO. At least, not entirely.
The idea for THE TESTIMONIUM occurred to me when I did an Easter monologue about ten years ago, with Pontius Pilate dictating a letter back to Tiberius Caesar trying to explain what had happened during that fateful Passover in Jerusalem. I began to think and turn over the idea in my head: What if that letter really existed? And what if we found it today?
And that was it. The idea sat dormant in my head for several years, until one day during GCS chapel the first chapter suddenly sprang up in my brain. I frantically began writing on a piece of scratch paper I had in my Bible (sorry Trent Brown, I may have missed the gist of your message that morning). At first, I recorded nothing but names. "Joshua Parker" - a combination of the names of two seniors I was teaching that year, Josh Collins and John Parker. "Isabella Sforza" - my daughter Rebecca was (thankfully) leaving her TWILIGHT phase behind, but the name Isabella was indelibly engraved in my brain. "Sforza" was a powerful family in Renaissance era Italian politics that I knew from teaching world history. Giuseppe Rossini - Rossini from the composer, Giuseppe because, well, it's an Italian name I happened to know. Father Duncan MacDonald - Duncan was my sister-in-law's maiden name, and as for MacDonald - well, I was getting hungry for lunch, OK?
An earthquake. Damage to the ruins of Tiberius' villa. A secret chamber. I had NO IDEA where I was going with this. I knew that there would be a romance between Josh and Isabella, but I had no idea they would wind up getting married. I knew the scroll would be discovered, translated, and published - and I had a sinking feeling that it might be destroyed before it could be fully authenticated. But how I did not know.
But the following storylines I had no concept of until they just, well, HAPPENED as I was writing: the truck bomb, Rossini's death, Josh's parents coming to Italy, Luke and Alicia Martens becoming major characters, and Josh getting shot. In fact, I wasn't really sure if that was how it would shake out until I was writing the penultimate chapter and that HORRIBLE cliffhanger popped into my head - you who have read the book know the one I am talking about, I have gotten actual death threats over it!
In short, I had a vague idea where the story might be going, but at the same time, I had no idea how many twists and turns it would encounter along the way. I'm glad I kept writing them down as they popped into my head, and I hope you enjoyed reading them as much as I loved writing them.
Right now I am about two thirds of the way through my fourth novel, LOVER OF GOD, and I have no idea how it's going to end. But I intend to keep writing and find out!