I wish I had a nickel for every person who's told me, "I'm going to write a book someday."
It's a harmless fantasy, and I'd guess that for every hundred who say that, one or two will actually do it. I'll leave the subject of publication for another post and consider actually writing the book that hovers in your mind.
It takes a great deal of will or initiative or whatever you call it to complete a book. The idea is there, and it's absolutely brilliant. It's that nasty time from in your head to on the page that kills most projects, why many give up after a chapter or two and most never even start.
The story won't go into motion. The characters hide among the words and won't show themselves the way you want them to. The plot doesn't thicken, or it thickens to the consistency of a cold glue stick. Those clever phrases that danced in your head in the idea stage become plodding dialogue and hackneyed prose as your fingers try to capture your interior brilliance.
The actual writing process requires force of will, and only the author's own hard work will do. That's why writing is a lonely profession: friends can admire, family can encourage, editors can demand, and publishers can tap their watches to warn of looming (or passed) deadlines, but only the author can summon the will to write, to keep writing, to finish.
Sometimes you apply the B.I.T.C.H. axiom (butt in the chair, honey!). Sometimes you give yourself quotas: so many words/day or pages/week. Sometimes you give yourself X amount of time off. And sometimes you write, write, write, even if it's wrong, wrong, wrong. Writing itself can restart the creative process, and you might find your way to something worthwhile.
So when people tell me, "I'm going to write a book someday," I smile and in all sincerity wish them luck with it. Luck is more for getting published than for writing, though. Writing takes will, as in "I will finish this book."