I think the first thing to understand is that screenplays and novels are two different formats.
Yes, there are a lot of similarities between the two, such as an active protagonist; an intriguing plot that has a beginning, middle, and end; obstacles for the main character to overcome; among others.
But, the biggest difference between the two is that screenplays focus only on what can be seen and heard. Of course some screenplays utilize voice overs to varying degrees of success, which is similar to hearing a character's thoughts in a novel. While novels revolve around the main character(s)'s head. Meaning, that the novel focuses on the main characters thoughts and emotions. When writing a novel, you really want to get into the main character's head and make the reader feel what the main character feels.
The best advice I can give is to find your favorite novel and read it. Read it for pleasure, then read it again and again. And each time you read it, look at how the author handles character, plot, description, action, etc. Because, again with screenplays, all the description for the scene is given right under the scene heading, whereas if you were to do that with a novel, it'd be considered an infodump, so you need to be able to seemlessly fit it in with the rest of the story.
Then once you've broken down one novel like that, do it again. Find another favorite novel, or one in the genre you're writing in, then find a novel in a different genre. Break each novel down. You don't need to write detailed notes on what the author did, just try to keep track of it while reading. Try to be more aware of what the author is trying to do.
Figure out what you like and don't like about the book. More importantly, figure out why you do or don't like something about the book. And apply those likes and dislikes to your novelization.
On the plus side, since you do have the screenplay, you should have a good grasp of the characters, plot, world, etc. to work from and you won't have to start from scratch.
If you want to read some books on the craft of writing, I'd recommend:
Story: Substance, Structure, Style and The Principles of Screenwriting by Robert Mckee
The Art of Fiction: Notes on Craft for Young Writers by John Gardner
Both these books came highly recommended to me by my creative writing instructors. They both talk about the elements of highly successful stories, though one is geared more towards screenwriting and the other more towards novel writing, but the principles remain the same.
The other thing to keep in mind is that since screenplays and novels are two different formats, the novelization of the screenplay will not and cannot be the same as the screenplay itself, not if you want to have both a successful screenplay and a successful novel. Just look at all the novels that have been made into movies and all the novelizations of movies. But what remains the same is the heart (sometimes the theme or the underlying idea) of the story that you want to tell.
Yttar
