From the perspective of a screenwriter, what is essential in creating an adapted script? Is it possible to keep the true essence and theme of a piece of literature when translated to film? Can literary techniques be directly transformed into cinematic terms? Should the two even be compared?
Sure. Books and movies should be compared, if only to understand what each does well.
Using words alone, a good book manages to evoke images and emotions in the reader that add up to a coherent story. The best writing makes a reader feel like he’s seeing, hearing and touching what the character experiences, putting you "in his shoes." Of all the literary tools available to the writer, the most valuable may be insight. The novelist can choose to tell the reader what the character is thinking, or fill in extra details, or sketch out relationships, that have nothing to do with the current scene. In fact, the novel doesn’t need to have "scenes" at all. Moments and observations can float freely in space and time, arranged in whatever order best suits the story.
A movie — and by movie I mean what’s actually projected on the big screen
— has basically the same goals as a novel. It wants to transport the viewer
into a different place and time, making him feel like what he’s seeing and
hearing is real. A movie has many advantages over a novel. Not only are there
concrete visuals, but you hear the characters speak and watch them fight.
It’s an exaggeration to say that a picture is worth a thousand words, but it would be very hard to capture the essence of THE MATRIX’s bullet-time on paper without having seen if first, or the feeling of a John Williams soundtrack. But this efficiency comes at a cost. With rare, art-house exceptions, movies have scenes. The viewer is seeing and hearing something that is taking place at a specific time and location. Movies move relentlessly forward at 32-frames per second, and the viewer cannot choose to stop and think about something, or flip back a few pages to catch something he missed.
Most importantly, movies lack insight. Aside from an occasional voice-over or narrator intrusion (done recently, and effectively, by AMELIE and Y TU MAMA TAMBIEN), a movie can’t communicate anything to the viewer beyond what is seen and heard. Since a movie can’t flat-out tell you what the hero is thinking, it has to be very specific with its images and sounds to let you know what’s going on inside a character’s head.
Now for the terrifying truth: a screenplay is the worst of both worlds. It’s a work of literature that has to conform to all the limitations of a movie, yet without any of cinema’s special abilities. That above all else is why screenwriting is so hard.
In terms of adaptation, the screenwriter has to look for ways to take ideas that "float" in a novel and tie them down to specific moments, locations and times. Sometimes this means simply repurposing internal thoughts as dialogue, but more often it involves a fundamental rethinking of the structure, storyline and characters to achieve the goal.
I think one reason that many adaptations rely on voice-over is that the filmmakers never found a way to externalize the essence of the novel they were adapting. Instead of making a movie that could stand on its own, they created the cinematic equivalent of a book-on-tape. To me, these movies always "feel" written, a huge limitation.