Adapting a literary work for cinema is in many ways like translating poetry. The form as an artistic artwork necessitates the translation of sentiment, rather than literal, semantic meanings, leaving the produce as an independent artwork.
The difficulty in translating the imagery of words into forced, visual pictures was, I feel, done particularly brilliantly in Orson Welles' 1964 adaption of "The Trial". The human mind works in peculiar ways, replacing the holes in what we experience with learned or imagined images. In Kafka's novel, the literally kafkaesque scenes are construed with faces, garments, colours, patterns and constructs from on our cryptomnesiac fantasies. The metaphor in language is then, in the motion picture, translated into the director's subjective experience of the novel. What interested me was how Kafka's narrative as a supple construct was moulded into an independent artwork. Depersonalising the experience of the novel, leaving holes in the reliability of the protagonist and the continuity of the plot, the film effectively conveyed the same feeling, the lack of reality and logic, and most importantly -- left the viewer with the ability to construct the same images in place of the holes, just as it is done in a novel. Each image contains the same particles of meaning as words and activate the same comprehensive, narrative responses in human minds. In the case of both literature and film, it is left to the audience to decide whether they take the information at face value or not. In "The Trial", however, the narrative of both novel and film seems to deliberately lie in order to support the irrationality of the plot.
Every time I watch a film it strikes me how costly it must be to put into real life the pictures you've imagined, and in this sense, how easily made a literary piece is made -- basically you only need the gift of putting together sentences. (The quality of such is a different discussion altogether). You don't even need to be able to think of a plot (I'm looking at you, Shakespeare). But returning to the translation of poetry, the delicacy of words proves an eternal challenge to put into a different language with its infinite phonemes and connotations -- or equally -- powerful, visual images.