If you read novels that were turned into movies, you can see how the movies (usually) alter the story to make it stronger. One way to do this is to keep the hero actively trying to solve his or her own problems and putting the people who have the most to lose in the most jeopardy.
“The Martian” largely follows the novel. the main difference occurs in the end. In the novel, the hero rides an escape capsule into space and remains strapped in while another astronaut floats out to rescue him. Yet visually that’s too passive and uninteresting.
That’s why in the movie version of “The Martian,” the hero unstraps himself from the escape capsule and cuts a hole in his spacesuit so the escaping air can act like a thruster and guide him closer towards safety.
In the novel, another astronaut reaches out to save the hero, but in the movie, the captain of the Mars ship risks her life to save the hero. By making the captain rescue the hero, this not only puts the captain at greater risk (since she’s more of a major character than the other astronauts), but it also helps the captain deal with her guilt in leaving the hero stranded on Mars in the first place.
Non-fiction books are even tougher to adapt into exciting movies because real life rarely condenses action but usually rambles and wanders all over the place. In the book “The Big Short,” the author describes the different major characters who all made money shorting the real estate collapse in 2008.
While the book can set aside the narrative to go into a detailed explanation of financial terms, the movie can’t. So the movie version of “The Big Short” has celebrity cameos where Margot Robbie takes a bubble bath and explains a financial term while Selena Gomez plays in a casino and explains how the collapse of one part of the financial market had a domino effect on the other parts of the real estate market.
“The Big Short” has to make dry, technical information interesting in a visual manner so there’s nothing more interesting than seeing celebrities in odd situations, explaining the technical information with real-life examples.
The main point is that books are often loosely structured, which means if a screenwriter closely adapts a book into a movie, it’s likely to feel flawed. That’s why screenwriters must tighten the story to keep the hero active and the obstacles logical.
In “The Scorch Trials” (the sequel to “The Maze Runner”), the screenwriter wisely dumped a large part of the book’s early scenes. In the book, the hero is trapped in a dormitory that seems safe, but suddenly comes across the dead bodies of their rescuers hanging from the ceiling. The next time they look in the room, these hanging bodies are gone.
This is never explained in the book so putting it in the movie would be equally confusing. Read “The Scorch Trials” and watch the movie and you’ll see that the movie deviates greatly from the book because the book is all about throwing strange and frightening obstacles in the hero’s path without ever explaining where it came from or why it’s even there.
For an exercise, take a book such as a classic and adapt the story into a screenplay of your own. “Clueless” is basically telling the story of the novel “Emma” but in a modern world so feel free to take any classic (it’s in the public domain so you don’t need the rights) and reimagine it in a different setting.
“Apocalypse Now” was based on the novel “The Heart of Darkness” so let your imagination roam free. You don’t even have to write the complete screenplay. Just jot down a rough outline of a story based on a classic novel and you may be surprised at how borrowing a novel can help you create an interesting story in a different setting.
More importantly, studying a novel will show you that novels and non-fiction books are often too loose and unfocused to be turned into a screenplay without some judicious trimming and alterations to make the story tighter and more logical.