Determine whether the novel you want to adapt contains enough stimulating visual content. For example, are there lots of action sequences in different locales or are the characters mostly engaged in long conversations/reminiscences around a kitchen table? Film is all about action, movement, and contrast - not about a succession of talking heads in a static setting.
Identify who the target audience would be if this book were turned into a movie. Does the subject matter have widespread commercial appeal or is it on a topic that has a fairly narrow focus? Consider movies you've already seen that are similar to the one you want to write and jot down notes of what made them so memorable.
Identify whether the book is plot-driven or character-driven. To be successful, a movie's storyline needs to be driven by the actions of characters who are forced to confront their internal fears/flaws and make choices in response to an external threat. In turn, the players undergo a character arc - the transformation from who they were at the start to who they become by the end. It's their own actions that push the story forward. In contrast, the characters in a story driven by plot are in a reactive - as opposed to proactive - mode. Natural disasters and war epics fall into this category because it's the event itself that is calling all the shots.
Identify the core conflict of the novel. This can be a challenge in a 400+ page novel and/or one that spans multiple generations. The reason is that the average length of a screenplay is 110 pages and must establish within its first 10 pages the central conflict that will drive the entire story. You also need to ask yourself what is original and filmworthy about the core premise. For instance, "Will Joe win Cynthia's heart?" isn't as unique as "Will paraplegic Dave overcome his fear of sharks and compete successfully in the surfing competition?"
Identify the protagonist and the antagonist in the novel. It is generally through the protagonist's point of view we learn what is at stake and what risks will need to be taken in the pursuit of reward, revenge, or escape. Equally important is an adversary who will stop at nothing to thwart the hero's attempts to reach his goal. If the novel is missing either of these elements, it may not be a good candidate for adaptation.
Break the novel down into a one page synopsis with a definitive first act, second act, and third act.
Research whether the subject matter has been done before in a movie. A good place to start is with the Internet Movie Database which allows you to conduct searches on plots (i.e., vampires) as well as appearances of historical figures (i.e., George Armstrong Custer) in previously produced movies.
Determine whether the rights are available to adapt an existing work to the medium of film. If, for example, it is someone's novel other than one you wrote yourself, you will need to contact the author, the author's agent, or the publisher and inquire about the film rights. For unpublished journals written by your own relatives or ancestors, you only need to ensure that the material in question won't harm or slander individuals close to the subject matter. Books that have long been out of circulation are easier properties to adapt to a new medium, but unfortunately, also beg the question, "So why hasn't anyone pounced on this already?"
Be prepared to pay a price for the right to adapt someone else's novel within a specified period of time. This is called an option agreement and basically means that the writer is "renting" the project to you with the mutual understanding that no one else will be given that privilege as long as you are working toward turning it into a polished script. An entertainment law attorney can walk you through the process or you can go to resource sites such as Canadian FIlm Maker and download the appropriate forms as Adobe Acrobat files.
Take a screenwriting class at your local college or invest in some screenwriting how-to books if this is new territory for you. Publishers such as Michael Wiese Productions offer a full array of reference books specifically targeted to film and television.
Purchase a screenwriting software program such as Final Draft. Screenwriting follows very strict formatting rules and you want your finished product to look as professional as possible.
Start your screenplay in the right place. The right place isn't necessarily going to be the very first scene in the book, especially if it's a book that's slow to get off the ground. Start your screenplay with an inciting incident that will immediately hook an audience's attention or tease them with a provocative question. Compared to the book itself, this particular scene may not happen until page 135, but that doesn't matter - what matters is a strong opening that sets everything in motion.
Think "minimalist" when writing. Unlike a novel that can spend pages and pages describing the details of a woodland vista, a screenplay is all about brevity. In other words: EXT. FOREST - MORNING. Everything else is left up to the discretion of the director, the cinematographer, the set designer, etc.
Write only that which is filmable. While a novel can go to great lengths telling us what a character is thinking, imagining, pondering, remembering, etc., we can only know these things in a movie if these thoughts are spoken out loud or conveyed through physical actions, facial expressions, and/or nuance.
Understand that dialogue in print is not always going to sound exactly the same as dialogue spoken out loud. Characters in novels tend to speak more eloquently, use less slang, and talk in complete sentences. Don't think for a minute you can simply lift all the conversations verbatin from a novel and use them in your screenplay as is. Recruit friends to read your lines out loud for you so you can evaluate whether it sounds natural or stilted.
Eliminate extraneous scenes from the novel that don't advance the plot or reveal new dimensions of character. In a 400+ page novel, this means that almost 300 pages of "stuff" won't make it to the silver screen.