Sometimes when people start to write something (this is something I've heard  many, many times), "I hesitate to get started because I don't want to compromise what's in my head." 

I worked on Jarnsaxa Rising for a while yesterday. I printed out my outline and carried it to bed with me, re-read it, and re-read it again while the coffee brewed this morning. I have a clear idea for the next scene, but it's not going well.  I took the page for the scene I was working on and rewrote the outline in what I call "turns:" X wants Y but hits obstacle A, D wants N but hits obstacle Y. And then I write short sentences of how those stumbles toward goals over or around obstacles succeed or fail. 

As I was scrawling this, I came to what I thought was a weak end. In my head it was this:

ERIC: Why didn't you tell me?

SIF: You had to find out for yourself.

(Cue "Somewhere Over The Rainbow." It's the Wizard of Oz all over again.)

As I was scribbling, it cleared up, and I was pleasantly surprised. This came out. I'm transcribing my scribbled notes. 

ERIC: Why didn't you tell me? 

SIF: I can only make you grow, but not change, either you choose your form or something else chooses it for you. (Jarnsaxa has to learn this.) When a tree grows, its leaves turn toward the sun and bend with the wind, roots go to water. 

The line, "its leaves turn toward the sun" came from a theatre design class I had as an undergrad. We were talking about how if you design a forest, you have to consider that the sun will cause that forest to look a certain way, you have to take science into consideration, not just "what looks cool on a stage." 

My point is, you might think you know in advance every detail about your story, and that's good, you should. But there is a lot of room for discovery in the process of pencil, paper, scribbling and time, and those discoveries will improve what you think you have now. 

EDITED TO ADD: Sassy thinks that if she sits in my lap while I type, my job is to gently pet her. If I type, she bites my hands. BYE SASSY.