Writing is a strange process. Yeah, I know you already know that. But, to the uninitiated, when you hear that someone is taking a year, two years, ten years to complete a book, you think, what a dilettante. May or may not be true, but it sure does seem that way. Until you try it yourself. An idea might come to you, like a muse visiting and leaving a gift. (Ah, the muse---- another time) Some lucky ducks take that seed of an idea and weave it into some lovely tapestry, or at least something they can sell. Others of us get a quick start and then stall. And stall. Characters, plot, narrative thread. Instead of telling a story like you would around the dinner table or over a few beers in the local tavern, you hem and haw and think, no, there's a better way to start. Have him say this and her do that and make sure it all makes sense and makes the reader want to turn the page. And then the words go silent.
Silent is where I end up too much of the time. Maybe I have mentally beaten up my characters , asking too much for their narrow shoulders. I have to back off. I have to let them behave naturally, as the human beings they purport to be. And, for me, that takes time. I have to leave room for them to tell me what they want to say and who they reveal themselves to be. Sounds a little tales from the crypt, but the more you write the more you realize you have to be an instrument, a vehicle, let the story happen. Unless you are writing a legal brief or a propaganda pamphlet, and even then you assume a certain voice. In writing that falls under the genre of "Creative", whether fiction or non-fiction, the writer has to get out of the way. Otherwise the reader will know that the story doesn't ring true.
We (a group of writing friends and I) have discussed the difference between the person who writes and the writer, or author. There is a different persona once you commit these thoughts to the page (or screen). The 'writer' is a creation and is not necessarily the one who looks a lot like you and goes about the daily business of life. There is a difference in that person once he or she starts writing.
I think we stall ( I assume that I am not the only one) when we think too much. One of my favorite expression is: Over analysis leads to paralysis. Sometimes, we just have to step aside and let the words come.