CHALLENGE NUMBER ONE - LENGTH
Screenplays rarely run longer than 120 pages. Figuring one page of a screenplay equals one minute of film, a 120-page screenplay translates into a two-hour motion picture. Much longer than that and exhibitors lose a showing, which translates to fewer six-cent boxes of popcorn sold for $5.99 at the refreshment stand. It took the author of your source material 400 pages to tell the story. How can you possibly tell the same story in 110 pages, the ideal length for a screenplay by today's industry standards?
And the answer to this question is no joke. "You can't! Don't even try!"
Instead, look to capture the essence and spirit of the story. Determine the through-line and major sub-plot of the story and viciously cut everything else.
By "through-line" I mean, WHO (protagonist) wants WHAT (goal), and WHO (antagonist) or WHAT (some other force) opposes him or her? It helps to pose the through-line as a question.
"Will Dorothy find her way back to Kansas despite the evil Wicked Witch of the West's efforts to stop her?"
The same needs to be done for the major sub-plot.
"Will Dorothy's allies achieve their goals despite the danger they face as a result of their alliance?"
One workable technique is to read the book, set it aside for a few weeks, and then see what you still remember of the story's through-line. After all, your goal is to excerpt the most memorable parts of the novel, and what you remember best certainly meets that criterion.
In most cases, everything off the through-line or not essential to the major sub-plot has to go. Develop your outline, treatment or "beat sheet" accordingly.
CHALLENGE NUMBER TWO - VOICE
Many novels are written in the first person. The temptation to adapt such, using tons of voiceovers, should be resisted. While limited voiceovers can be effective when properly done, remember that audiences pay the price of admission to watch a MOTION (things moving about) PICTURE (stuff you can SEE). If they wanted to HEAR a story they'd visit their Uncle Elmer who drones on for hour upon hour about the adventures of slogging through the snow, uphill, both ways, to get to and from school when he was a kid, or perhaps they'd buy a book on tape.
The old screenwriting adage, "Show, don't tell!" applies more than ever when writing an adaptation.
CHALLENGE NUMBER THREE - "LONG-THINKING"
Some tribes of American Indians had a word to describe those of their brethren who sat around thinking deep thoughts. Literally the word translated to, "THE DISEASE OF LONG-THINKING". Quite often, lead characters in novels suffer from this disease.
"Mike knew in his heart that Judith was no good. Yet she caused such a stirring in his loins, he could think of nothing else. He feared someday he would give in to this temptation named Judith, and his surrender would surely bring about the end of his marriage!"
If adapted directly, how on Earth would a director film the above? All we would SEE is Mike sitting there, "long-thinking". That is not very exciting to say the least. And as mentioned previously, voiceovers are rarely the best solution.
When essential plot information is presented only in a character's thought or in the character's internal world, one solution is to give this character a sounding board, another character, to which his thoughts can be voiced aloud. Either adapt an existing character from the novel or create a new one. Of course as always, you should avoid overly obvious exposition by cloaking such dialogue in conflict, or through some other technique. Even better, figure out a way to express the character's dilemma or internal world through action in the external world.
CHALLENGE NUMBER FOUR - WHAT STORY?
Mark Twain is quoted as saying about Oakland, California, "There's no there, there". Similarly, some novels, even successful ones, are very shy on story and rely for the most part on style and character to create an effect. Some prose writers are so good at what they do, that their artful command of the language alone is enough to maintain reader interest. Such is never the case in screenwriting.
Successfully adapting a "no-story-there" novel to screenplay form is a daunting task. One approach is to move away from direct adaptation toward, "story based upon". Use the brilliant background and characters created by the original author as a platform from which to launch a screen story. In fact, if for any reason a screenplay doesn't lend itself to screenplay form, consider moving toward a "based upon" approach, rather than attempting a direct adaptation.
Congratulations! You're now an expert on adapting novels to screenplay form! Well maybe not an expert, but hopefully you have a better understanding of how to approach the subject than you did ten minutes ago.