Mulling over scenes for the Ginger's Gift screenplay

I've been mulling over scenes for the Ginger's Gift screenplay in the last week. While writing a screenplay and making a movie has been on my list of things that I have wanted to do since I was oh, probably 13, this is the first time I'm attempting it. I have been reading the "Marley & Me" screenplay to use as a guide and it's interesting to see what was supposed to be shot and what actually turned into the movie. What I struggle with most as I do this, is how do I make it as close to the book as possible? I always see how books change as they are made into movies and now I'm understanding the true difficulty of this because it needs to be a powerful screenplay yet be true to my story. More to come as I continue to form and mold my notes into a piece of work.