Answer by Jon Ferreira:

Overview
Writing a screenplay or a play from source material like a book or even real life requires a significant amount of editing and cutting material. As opposed to a novel, screenwriters just don’t have the page length to explore characters’ extensive backgrounds, elaborate settings — nor do they have the luxury to include a cast of thousands (or hundreds – or less) all of whom have a penchant for endless verbosity. There just isn’t the time in a two-hour film. Here are some of changes screenwriters make and why:

Characters
Books typically have way too many characters since the novel is typically trying to capture the reality of life. Real life has 7 billion people on the planet, and the average person meets 12-50,000 people in the course of their life. Although you really only need 3 characters to write a traditional story most fiction books average at least 20-80 characters. Some books by Tolkien, Martin, Dickens, and Tolstoy, sometimes include casts of hundreds. Because a book has narrative that can take its time, and spend time on each of these characters, there’s no reason not to write that many. In Marquez’s One Hundred Years of Solitude the story spans six generations, and in each generation, the men of the Buendía line are named José Arcadio or Aureliano and the women are named Úrsula, Amaranta, or Remedios. There are dozens of characters, and it is confusing enough to follow the relationships on paper where there are names and it’s written in front of you. Try putting that complex family tree on screen where characters aren’t wearing name tags, and it becomes confusing quickly. People watch movies and tv to escape and to watch something they can follow. Nobody wants to be confused and lost for a whole movie.

Cutting Characters
Consider the show Game of Thrones. In the books, George RR Martin writes an extremely dense story, with a complex plot of interconnecting characters –kings, queens, knights, servants, clergy, dragons, etc. Each character belongs to a different House, like House Stark, and you are told what knights are loyal to House Stark, what commoners are, etc. You learn the whole history of that one house, and all the ancestors. Each have their own names and histories. There are dozens of houses! Martin introduces characters he may never bring back, JUST to fill out his world realistically and create a fantasy atmosphere. Tolkien did it too with the Lord of the Rings series. Of course, the average book is about 1000 pages. The average screenplay is about 100. And much of those pages describe action, not dialogue. Martin’s books simply have too many characters. The show, as it is, can sometimes be hard to follow. And they cut out hundreds and hundreds of characters.

It simply would be too confusing to include every character in a book. They would have to walk on and walk off if they weren’t the main characters, because there’d be no time for them to speak. We wouldn’t get to know them. Additionally, it costs a lot to cast a film of hundreds.

Compounding Characters
Sometimes we need the information or action that a particular character has or does. Perhaps we need the same from another similar character. Instead of including both, a screenwriter will conflate the characters into one character and take one of their names. This is a way to keep some of the plot but cut many of the characters.

Inventing Characters
Sometimes it is necessary to invent characters, in order to play off the main characters. For instance, maybe in the book, the character thinks to himself about what he’s going to do. That wouldn’t work in a movie, since most films try and stay away from overusing voice over. It simply lacks action, and can be boring. Instead, maybe they invent a confidante role, who is the best friend whom the protagonist tells all his inner feelings to. Maybe the story needs another villain or a funny character to lighten the mood. Characters are certain archetypes — the mother, the fool, the lover, the soldier, etc. Screenwriters sometimes need to add a certain type to get the movie working.

Streamlining Action
Often the plot of a book unfolds over several hundred pages, allowing for dozens, even hundreds of scenes, each with their own plot point. If we consider the length of a script, it’s necessary to cut MOST of those scenes. There’s no way a movie could try to show hundreds of different things that happen. Each scene would have to be about 20 seconds long. The writer must choose the scenes which advance the story the most, and are vital to making the story arc make sense. The scenes should fall into the general structure of a play/screenplay…inciting action, rising action, climax, falling action, denouement. Scenes that slow the story down are cut, however good they may be in the book. The pacing and movement of the plot are everything in a movie. That means action over verbose talking and deep exploration. Think of it like a movie is the cliff notes of a book. It just covers the surface of the major plot points and no more. Good movies use powerful imagery, color, costumes, sound, lighting, and cinematography to tell a story as much — if not more — than the words. In film, a picture is worth a thousand pages.

Voice
A book series like The Song of Ice and Fire (Game of Thrones) has at least a hundred different characters leading chapters, where the narrative is from their point of view. We hear them speak, and hear their thoughts as well. In a movie, typically a screenwriter will choose a clear protagonist, and we will see the world of the film from his point of view. If there’s voice over, it will be his. We will see action and character only through his eyes. This isn’t always the case. In Game of Thrones the show, the story will show that character’s scenes, and they are sort of the protagonist of those scenes, but I couldn’t tell you who was supposed to be the protagonist of the entire show.

Budget Concerns
By its very nature, a book has the ability to describe ANYTHING. Things that don’t even exist in the natural world. Things that have not been invented yet. Explosions that are epic and battles between gods that shake Heaven and Earth. Up until recently, movies haven’t been able to show all of those kinds of things, and action as realistically as real life. There were only models, and fake sets, and budgets were only so big. A movie like Lord of the Rings — as close to its description in the book — could not have been made even 20-30 years ago. Now, with CGI, seemingly anything is possible. But that’s not always true. Some things that are called for can’t be done with CGI, and other things are just too expensive. If a movie can’t afford all the stuff from the books, they pick and choose, and do what they can.

Show, Don’t Tell: Adding Action
On the flip side of the budget concerns, is the frequency of Hollywood to ADD action, to make a film more exciting. The recent Sherlock Holmes movies don’t resemble the books at all, because they have made Holmes into a big action star, and have filled the movie with fights and explosions. This happens in nearly every adaptation, in order to make the stories more interesting. Books have pages of monologues and inner thoughts, and those are boring on screen. The general rule is show, don’t tell. Don’t have characters talking endlessly, when you can visually show it. Action is much more physical and visceral than in books. The audience would rather see action, and sometimes that means making it up.

Character Complexity
Not only are dozens/hundreds of characters cut out of a film, the characters that are left, are not nearly as interesting as they may be in the books. In a book, there’s a whole lot of text about the inner life of a character — what he’s thinking, feeling, planning, etc. This kind of thing ends up being boring on screen. That means characters aren’t as complex and deep as they are in a book. We don’t know them psychologically as well as we do in the novels. However, since we can see them, we know them physically much more. Ever wonder why Hollywood is so obsessed with skin-deep beauty, and books tend to be more cerebral? It’s the very nature of how they are made.

Locations
In books, the action of the story can take place in dozens, if not hundreds of different locations — both indoor and outdoor. On top of a mountain and deep under the ocean. CGI technology has helped with green screens able to project almost any location, but sometimes it just is no substitute for the real place. Movies may not have it in their budgets to go on location or CGI it in. The other consideration is that sometimes the book will have to be cut so that two scenes that weren’t next to each other in the book, are in the film, and may need to be in closer proximity — the dining room and the bedroom in the country estate, as opposed to the dining room in the estate and a table at the restaurant. Locations are streamlined, for continuity sake.

Rearranging Order
Sometimes a book simply isn’t written in a way that makes narrative sense in a movie. Perhaps it’s told out of sequence. Perhaps the climax happens to early in the book. Maybe characters are introduced too late. Movie scripts generally have a formula which they stick to — Act I, Act II, Act III — and each has important elements that are supposed to happen during them. Books don’t follow such logic. They tend to be more stream of consciousness, meander more, are verbose, take pages to describe something or someone, and completely mess with traditional narrative and plot development. Books are from many genres, and writers are sometimes experimental and avant garde. Despite being the newer art form, in many ways, film is much more traditional and formulaic. To some extent, screenwriters have to be merciless, and cut and arrange as they feel it serves the film.

Tweaking Time
Often for the sake of telling a story that may have begun as a 1000 page book, in less than two hours, a screenplay will have to collapse time, and tighten scenes into a logical and more reasonable narrative. This is one of the major complaints about movie adaptations. They often compact time, so that something that happened over the course of 10 years, happens in just nine months. The time it takes to get certain places may be shrunk as well. Famously, in Star Trek, even using their space age warp technology, the starships should take years to go the distances they go in just days in one episode. Of course we don’t want to watch a ship’s entire voyage. In ASOIAF, we’ll see a voyage take weeks or months. On the show, in one scene they’re in Point A, and in the next scene, they’ve already arrived at Point B. Time is compacted. For the sake of keeping things interesting, movies and television always compact or lengthen time, depending what the script needs.

Capturing the Spirit
Ideally, a film will keep as much of the story, and cut as little as possible. Realistically, a movie is a very different medium, with different demands and characteristics. A screenwriter must necessarily cut, rearrange, conflate characters, cut characters, change locations, change the date and setting, or whatever else needs to be done, in order to serve the film. NOT the story. The FILM. A movie is a new work of art interpreted by filmmakers, and based on another medium’s work of art. They can never be identical reflections of each other. However, they can be complimentary works of art, that resonate off each other. They both have value and strengths the other doesn’t. A film is a visual art form, and will always value aesthetic over language. Images are its language. The most important thing a screenplay can do is capture the spirit and tone of a book, and if not every plot point, the major ones needed to resemble the original story. It’s about honoring the writer and trying to maintain the integrity of the book, while making it uniquely a film.

When making a movie from a book, why do they leave some/most of the book out of the movie?