I've been working on my novel, Medusa's Daughter. I got the first draft of chapter four (almost) done last night. It went well, but it was the first chapter from the point of view of the lead male character--Nikander. I have a huge outline that is very detailed, but I had to change things once the chapter got going. I needed to add characters to make things work. It was a big change to the outline that needed to happen. Nikander could not be alone to back up a major point in the first three chapters. I didn't realize that until I got to writing chapter four.
I learned from a book on writing called How to Write A Damn Good Novel by James N. Frey that sometimes you have to "redream the dream." You have something in your head for a while, but when you actually start the writing you realize it doesn't work.
My message is: even if you have a detailed vision of a book, be prepared to change it. Redream the dream. Throw out what you had originally thought. Change it. Read: How to Write A Damn Good Novel for some great advice.
Good luck with your own writing!