His whole weird, stubborn life, that wonderful crank JD Salinger refused to let Hollywood get its meathooks into Catcher in the Rye. Part of this stalwart aversion is owing to a bad experience when his story "Uncle Wiggily in Connecticut" was turned into My Foolish Heart, but JD also maintained that his writing was unfilmable, unactable. This resistance story on its own has a tinge of pretension to it, but a sterling core can be found, I think, in Salinger's refusal to have Holden Caulfield pictured on any of the jacket art.
We get some idea of Holden's looks from the text--redheaded with a grey streak, mature-looking for his age, a backwards "people-shooting" hat--but otherwise, Holden's a bit of a ghost when I read him. He's animated feelings, opinions, and meanings. I think of Ignatius from John Kennedy Toole's A Confederacy of Dunces in about the same way. He's such a lumbering absurdity full-up with these rancourous fusty opinions that it's somewhat difficult to picture him up and moving in a real world of meat and bone. But Hollywood's been threatening to make this movie since it won the Pulitzer, originally envisioning John Belushi in the role, and over the years I hear that Chris Farley, Will Ferrell, and Zach Galifianakis have been in talks to man this impossible man.
Watching a movie is a considerably passive act compared to the active challenge of reading. Books come with assembly required. Don't get me wrong: films at their best demand an astounding amount of work from the viewer, an ability to harmonize what you're being shown, how you're being shown it, what you're being told, and how you're being told it. While all good art requires participation to create meaning, books alone require the animating spark. Or maybe it's more like disposing of a body: the author gets the head, and it's up to you to get the feet (I'm doing my best not to drag Barthes into this...). Movies tend to drag away and bury the body; the viewer kinda keeps a look-out.
There's a famous story from the making of Alien. Originally, said alien was to appear prominently on screen but dailies revealed that a guy in black leotards would snap the suspension of disbelief. The fix was to barely show the titular alien, relegate the creature to shadows and peripheral blurs. It's rare that an imagined fear will be lesser than a shown one. I'm wondering if the same thing can be said about a deftly wrought literary character.