Making a movie, a good movie, is an achievement just shy of a miracle.

Unlike writing a novel–which (if you have the talent) requires nothing but determination, a pen, and a passable vocabulary–financing and producing a feature film often requires the cooperation of hundreds of people, many of whom could care less about the artistic merits of the project.

Don’t get me wrong: filmmaking is not rocket science, it’s more akin to herding cats.  And, with the advent of digital cameras, producing a film is a hell of a lot easier to do than it used to be. Still, movie-making is and always will be a collaborative medium. Even if you’re a guerilla filmmaker and you shoot a movie on your iPhone in the subway on your way to work, you’ll need at least need a couple of your buddies to lock down the location and make sure some homeless person doesn’t walk into the shot. The incarnation of a story from the verbal to the visual becomes inherently limiting.

Which is to say, even a good screenplay, based on a good book, won’t translate necessarily into a good film. As much as people rant and rave about cinema being a visual medium, films are driven first and foremost by the performance of the actors. A good film requires a mystical ‘lightning-in-a-bottle’ chemistry of its lead characters.

Case in point: after his success with Good Will Hunting, Gus Van Sant gave the world the most pointless and the most interesting cinematic experiment with his mostly shot-for-shot remake of Psycho. Even with the words, visuals, and sounds the same as Hitchcock’s original, the film was lifeless. Anne Heche as Marion Crane and Vince Vaughn as Norman Bates just don’t work. Hitchcock notoriously regarded actors as nothing but mere cattle, but I think it’s clear the original Psycho’s success can be attributed largely to Anthony Perkins and Janet Leigh’s performance. If its success was all to due with Bernard Hermann’s score and the storyboarding of the shower scene, the remake would have just as much power as the original. But of course, it doesn’t.

With that idea in mind, let’s look at the first tool a screenwriter has in his arsenal when adapting a book to a film, which is simultaneously his greatest (and really, sole) challenge:

Time

Famed Russian director Andrei Tarkovsky posited that “the dominant, all-powerful factor of the film image is rhythm, expressing the course of time within the frame.”  Making a film, to him, was to sculpt a work of art in time.

The average feature film is 110 minutes long. Book reading is much less temporally regular, depending as it does upon the speed of the reader, but on average it takes about three times as long to read a book than watch its feature film adaptation. For example, watching the entire Harry Potter movies takes around 20 hours whereas reading the books takes 60. Likewise, the theatrical versions of The Lord of the Rings are around 9 hours combined, whereas reading the books takes around 26 hours.

Now, interestingly, considering how much I’ve written about The Hobbit so far in this series, “The Hobbit” movies are the only films with more minutes of film than book pages. Thank God for fan edits.

Television gives you more wiggle room. As it appears George R.R. Martin will never finish The Song of Ice and Fire series, the HBO adaptation will likely clock in at seven seasons and around 50 hours when all is said and done—just about half the hours it would take to read the entire series up until this point.

Because of this ratio, a screenwriter will largely be compressing and streamlining the narrative, while also enhancing, expanding, and/or reworking pivotal incidents merely hinted at in the book.

How does one decide what goes, and what stays? We’ll look at that next week.