Okay, we definitely were on a roll yesterday, and I'm going to try to keep it going. (Although I may very well fail, oh, god, the pressure, how am I gonna write something as comment-worthy as yesterday?)
So, according to your comments, it looks like we start our novels based on: the things that fascinate us, character and theme, putting a fresh spin on an old tale, theme, theme and story, situations, character, depends on the book, story, character and theme, more character and theme...
And when the writing is good, it all just builds on itself, doesn't it? It all seems so organic, because of course those actions would happen to that character based on that situation which underscores that theme... and it all just rolls together, and then on the page it's almost like seeing Athena, sprung forth fully-grown from Zeus's skull. Or, as Stephen King describes it, like uncovering the final pieces of the fossil and realizing that even the smallest bone from the beginning of your efforts really does fit into and support the whole, even if you couldn't see and understand the whole when you first got started.
So, where does that leave us, dear writers? If we start with something we love, something that drives us, whether it is a compelling situation or character or plot/story or thematic idea... can we go wrong, so long as we remember to bring the remaining elements to bear during the rewrite process?
Perhaps King was just warning us away from starting with theme as it overlaps with symbolism, rather than as it overlaps with character and story. One of the themes in my novel is the concept of identity... how do you know who you are and what you stand for? But another theme is that of naming... how are we represented and perceived based on the names and labels that others choose for us and that we choose for ourselves?
Identity might fall into the character-theme overlap. If I care about that moment where a person realizes who s/he is and who s/he wants to be,* then I will make certain character and story choices to bring out a tale that fascinates me (and hopefully, will fascinate readers).
Names, however, might fall more into the symbolism-theme overlap. If I start writing a book because I think names and their origins and meanings are ever-so-cool, and I definitely want to have the names of each of my characters mean something... well, that's kind of a non-starter, isn't it? Instead of being like the thin line between reality and fantasy in King's Bag of Bones, it's more like the use of blood in Carrie: something that "can serve as a focusing device for both you and your reader, helping to create a more unified and pleasing work" but not, ultimately, a basis for a novel, whether plot- or character-driven.
Where is the line between theme and symbolism? Are there symbols that you find especially moving? Are there any that are strong enough to carry a story, or are they just an element of the language craft?
Tell me more about what you write, and why it moves you.
* "...he had seen men come of age before and it always moved him. It was not a matter of their twenty-first birthday."
-- Hemingway, The short Happy Life of Francis Macomber