A playwright who used to write short stories told me how much he likes being able to leave it up to actors how to interpret a line of dialog.
A novelist who writes “He stared wistfully into space” may feel obliged to strike it out as a cliché, then try to come up with something more evocative. A screenwriter can leave this to the actor or director. An actor knows a lot of different ways of starting wistfully into space. But this is knowledge stored in a non-verbal area of the brain -- when I stop writing dialog and start describing a gesture, I have the sense of reaching for a different tool-kit.
Here are a couple of instances of body language from novels I read recently. The first is from Anthony Trollope's The Way We Live Now. Georgiana Longstaffe is under pressure from her family to break off her engagement to a Jewish businessman, Mr. Ezekiel Brehgert. In correspondence with Brehgert, Georgiana inadvertently reveals that her main motive for marrying him is that he has a fashionable London address, and in response he breaks off the engagement. Trollope is concerned to emphasize that Brehgert comes out of this business better than the Longstaffes – this might conceivably have gone over the head of a complacently anti-Semitic reader -- so he includes a scene where Brehgert, dealing with Longstaffe senior on other business, alludes to the broken engagement and comments “I think that throughout I behaved like a gentleman.” Here is Longstaffe's response --
“Mr. Longstaffe, in an agony, first shook his head twice, and then bowed it three times, leaving the Jew to take what answer he could from so dubious an oracle.”
When I visualize Mr. Longstaffe shaking his head twice and then bowing it three times, this seems entirely plausible. But when I try to picture an actor doing the same thing, it seems like over-acting. In a television production, you'd probably just get a close-up of Longstaffe's face at this point, and the actor would find a more economical way to do the heavy lifting.
This next example is from Malcolm Braly's On The Yard. In a flashback, Chilly Willy runs into a schoolteacher soon after having a one-night-stand with her.
“She was looking at the magazines racked on the counter of the cigar stand, and there was something in her posture that suggested to Chilly she had seen him first and turned away to avoid an encounter.”
This works because there's something distinctive about the way we stand when pretending we haven't seen someone. An actress could demonstrate this posture easily -- the schoolteacher is standing stiffly, over-intent on the magazines? But Braly does well not to provide more details than he does – the way he writes the scene keeps the focus on Chilly's realization that the woman is avoiding him, which is what's emotionally significant in context.