Adapting things like books, comics and video games into films can be a tricky process due to certain characteristics of one artistic medium not being easy to translate into another medium. This often leaves the film adaptations as lack luster shells of the original stories or incapable of conveying the same emotion and detail found in the original book. However, there are a few ways to get around many of the pitfalls that plague film adaptations on a regular basis.
First, one should read the material one's adapting and know where to make necessary cuts. This is the part of the film that fans of the book usually complain because you are cutting things out of the story. However, it is often essential to condense a story from a book that spans hundreds of thousands of pages into a two to two and a half hour film.
For example, "The Lord of the Rings" trilogy cut out the character Tom Bombadil as well as other details, such as the war in the north, in order to focus on the Fellowship and their quest to destroy the One Ring because that's what was truly important in the story and what needed to be focused on in the film. This lead to a more streamlined and focused story that could fit into a film.
However, the opposite problem can also arise. If you have a short book and need to expand it, only expand it slightly or you run the risk of desperately trying to stretch a thin plot over multiple movies. For example, "The Hobbit" trilogy was adapted from a book that was barely over 300 pages, but the story was stretched over three long films and added a lot of filler, such as a pointless love triangle and foreshadowing for the "Lord of the Rings" trilogy that has nothing to do with the story at hand.
Second, if you are adapting moments from a book that relied heavily on description and dialog, one should convey those moments in a film visually. One of the most common phrases used in filmmaking is "show, don't tell," due to the film being a visual medium rather than a written one. One of the best ways to shorten a book to film length is to convey certain moments in the book as short visuals cues.
Finally, when adapting a book to film, one should always keep true to the "spirit" of the original as much as possible. Small changes and cuts in the story can be tolerated as long as the filmmakers are able to convey what people loved about the original source material. On top of reading the original source material, filmmakers must reach out and ask fans of the material and, possibly, the author what the book is about and why these characters and scenes are important to get right on the big screen.
Are these tips guaranteed to make an adaptation perfect; not entirely. In fact, some adaptations can benefit from going completely "off book" if the original source was of questionable quality. The best examples of this are the adaptations of "Starship Troopers" and "Ready Player One." However, adaptations that follow these directions often work more than not and I feel that it's necessary to improve the overall quality of film adaptations that come out of Hollywood these days.