Screen writing can be stretched to cover many things. Put writing in quotes and the possibilities for sophisticated definition aggravatingly multiply. In its basic and most simple form, however, screen writing is dialogue, the words we hear spoken. That, at least, is how it is most commonly understood, and that's what I wish to deal with here. In truth, very little has been written about dialogue, so I can feel less apologetic about collecting some scattered thoughts about it.
Let's start by admitting that there are as many uses and effects of dialogue as there are contexts. If we could pinpoint a number of repetitive patterns we could perhaps reduce the number of contexts and try our hand at mapping out some systems of dialogue (given situation y, dialogue ? should follow). But that's a task I think is premature to undertake, so I'll be content to work with "instances" instead.
Some obvious observations- so obvious, apparently, that concern for them has been minimal indeed- should be made at the outset. Since the advent of the sould film, whenever we see human beings on the screen we expect them to talk. When they don't, our sense of a complete human being is violated (i.e., the mute girl in The Seventh Seal). Speech is a defining human faculty, so we expect dialogue: if they are human, they can speak. And we expect certain people (military captains, wealthy playboys, unshaven outlaws, ladies of the night, etc.), depending on their dress, age, the situation they find themselves in, and a bunch of other cues and factors, to speak accordingly. Hence dialogue is generally keyed to propriety, and this "law" makes it possible to exploit incongruity (Stan Laurel's mechanically learned mouthful to Oliver Hardy in the attic sequence of Sons of the Desert).
Despite all the arguments that have been made on behalf of film as a visual medium, the common viewer hangs on dialogue. We whisper, sometimes frantically, to our next-seat neighbor, "What did she say?" and really hope to get an answer, because we sense that a lot hinges on our knowing what was said. We squint to hear better. When nothing is said for a long time we can grow tense (the silent robbery in Ri fifi) or uneasy (the verbal silences in Antonioni as signs of dwindling human communication) or curious (the experiment of Russell Rouse's The Thief). Among the most troubling moments at the movies are when we are unsure whether words were spoken or not. We thought we saw the lips move, but something inaudible or only half-audible came out. Something resembling sound more than speech. We always want to understand, and what characters say to each other or to themselves is crucial to our desire for a firmer grasp of what's going on. And since film is a medium in which "writing" (i.e., most commonly talk) could be dispensed with, its appearance is always meaningful. A page would be blank without words, but the screen would still contain the meaning(s) of its images. Every instance of dialogue can be seen as a means of enhancement. It could be left out, but there it is- and the wish behind it is to improve on what's already there.*
While there may be a general tenor to the dialogue of a film, the dialogue from scene to scene is rarely consistent in the way that dialogue in literature tends to be. Film dialogue is incurably opportunistic, it caters promiscuously to each shift in context. No matter how tight the film's underlying structure, dialogue-as part of the surface whose job is to gain and hold attention-plays fast and loose above it. Thus, in a single film, there may be dialogue designed to make you think, dialogue that's twaddle, dialogue that's just "there" (pointless- or, better, unpointed- non-distracting, as "naturalism," perhaps), dialogue that's dense and rippling (as when some of the literate characters come together), sudden speechifying dialogue appropriate in a local, immediate way but incongruous with the rest of the film, dialogue with a moralistic undertow functional to theme or character indentification, and a minimum, probably, of 57 other varieties, including lines put in just to get a reaction, to create a small, sharp impact. …