1) The stage necessarily demands some degree of exaggeration: the more true-to-life actions seem to be, the less interesting they look to an audience.
2) The stage is the most receptive media for varying lengths: audiences are equally open and receptive when watching 10-, 30-, 40-, 60- or 150-minute plays. Paper is much more limiting – a 30- or 70-page story is not a short story, nor a novel, and won’t find readers easily. And what to say of film? If it’s not between 80 and 130 minutes, it’s either a short with no chances of distribution or a long-ass epic that will demand twice its production budget in marketing to convince people to watch.
3) The stage is more suitable for fewer and longer scenes: while we can’t ignore the technical difficulties and the time it takes to set up each new scene*, there is an aesthetical dimension to it as well. A scene performed live demands more time to be fully taken in by the viewer – if you watch a couple fight for two minutes and then move on to something else, you probably won’t relate to the scene, retaining just a look or a line or a gesture and trying to ask yourself “what the hell has just happened here?” before a new scene hijacks your attention and cuts your reflection on the previous one short. On paper, scenes can be long and overwhelming or short and unremarkable, with the narration adapting proportionally to let you know what to dwell on and what to take at face value. Film achieves a similar effect with editing.
*Though I remember a Brazilian production of David Mamet's Edmond that presented its 23 scenes in 65 minutes, never dimming the lights and never taking more than a few seconds between the end of a scene and the beginning of the next. The set was made of three fake walls on wheels and never more than two or three light pieces of furniture, which the cast itself dragged to different positions for each new scene. Gripping action, believable characters and clear plot with nine actors and an average scene length of 2m48s -- not bad, huh?
I can’t think of anything else about live drama that doesn’t apply to other medias as well: motivation and transformation are not more necessary onstage than in any other story, and superfluous action or dialogue (repetition, stated emotion, answered questions and filler words) is unwelcome and tiring in any story you try to follow. But it would be quite soothing to find more such traits to hang on to when we feel lost in our writing. If you can think of another one, I’m dying to hear it.
Also, I’m focusing on prose fiction and film as our most obvious language counterparts, and deliberately ignoring narrative poems, songs and radio plays. But finding differences and similarities between stage drama and those medias would be as helpful in defining the boundaries of theater, and I think even non-narrative art like instrumental music and abstract painting/photography could offer valuable insights in terms of rhythm, recurrence and spectacle.