A while back, Stephen Hunter wrote an essay for the Washington Post in which he examined the differences between fiction and Hollywood films taking fiction as "source." At one point he makes this claim:
. . .The primary issue in prose is motive: You have to understand why the people do what they do, or else the whole shebang falls apart as illusion. The minds of the characters have to be consistent to be believable; action has to flow from character. Fiction writing is about what happens internally, even if lots of guns come out and stuff blows up.
I know that a lot of people believe that this difference between the external and the internal is an important distinction to make between fiction and film, and that drawing it usually results in an implicit--or not so implict--valorization of fiction over film. Fiction gives us access to "the mind" of a character in a way film cannot; a corrollary of this is that the internal view is perforce a defining feature of fiction, that those stories and novels (particularly the latter) making a claim to be "serious" must provide it or risk being dismissed as not properly literary.
It is true that often the difference between a given novel and its adaptation to film is the greater focus on "mind" in the former. But this is a difference that is really only palpable when otherwise the novel and the film have much in common, when it was possible to adapt the novel to film to begin with because they both emphasize character, setting, plot in more or less conventional ways. (When finally the guns do come out and stuff does blow up.) In my opinion, the internal/external oppositon is not a very solid peg on which to hang one's hat in promoting fiction's putatively greater sophistication. We all recognize the ultimate tradeoff: immediacy in film vs. "depth" in fiction. But what makes depth the more valuable property? In aesthetic terms, why is it important to provide such depth in the first place? If you're more interested in "empathy" or "motive" than in art, perhaps.
What if you don't really care about "believability"? What if character is something you're not really interested in at all, except insofar as it enables the fiction's aesthetic design? Is such a writer (or reader) not being serious? What if it doesn't matter whether action follows from character? Couldn't character follow from action, if incident and event comprise the engine of aesthetic effect? Couldn't you dispense with action altogether? Couldn't you dispense with character altogether? (Think Beckett's later work.) Some writers want the "whole shebang" of illusion to fall apart. It's precisely a way to divert the reader's attention to some of the other aesthetic possibilities of fiction. Why is this less "serious" than writing the same old character-based story in which we get supposedly "luminous" glimpses into that abstraction called "human consciousness"?
Such fiction as I am describing would indeed be (mostly) unfilmable, at least according to currently reigning ideas about what film properly does as well. But wouldn't this be the whole point? We wouldn't need to have advice about how to approach a film based on a novel. They'd each have their own separate and entirely respectable jobs to do.