Some people are intimidated away from writing a book because they think we authors have the whole book in our heads when we start. Heck, most of us don't have the whole book in our heads when we finish. They think that it's all there, we write it down and we're done. LOL. Don't I wish.
Some of us (like myself) prefer to work with an outline. I've discovered that I like to work with a VERY detailed outline. Of course, I can change it (and I always do), but I know it's there like a security blanket. Other brave souls come up with an idea and just strike out on their own, no outline, no nothing -- they feel that to write anything down would sully the creative process. Most authors are somewhere in between. But all of us have one thing in common: we all have to write our books one sentence, one scene, one chapter at a time.
I absolutely MUST work this way. While of course I have my outline, when I'm actually doing the writing I have to force myself not to think much beyond the one moment in that scene that I'm writing. When the sheer enormity of what I have to accomplish pushes its way into my thoughts, my poor little brain just shortcircuits -- actually it panics. How am I going to get from here to there? Oh crap, I forgot to include that character. Do I really need that character? Should I save him and his subplot for the next book? How is that subplot ever going to fit in? In short, I try to do what I don't think any author can do -- have the entire thing in your head at one time. It's kinda like looking at deep space pictures from the Hubble telescope. Your jaw drops open at just how vast the universe is. The same is true (on a much smaller scale) of your books' universe. It's just too big to comprehend all at once.
And when you do that, you lose the immediacy of the sentences you're writing, the intimacy between the characters in that scene. You lose that emotional human (or elf or goblin) touch. The realness of two people who care about each other, or hate each other, or one is about to betray the other -- their intimacy/connection/animosity is lost unless you immerse yourself in their moment, get into their minds, and understand what they're feeling. Only then can you accurately convey your characters' emotions and make the words come to life on the page -- one sentence, one scene, one chapter at a time.