Q. In writing prose fiction, one key to vivid descriptions is specificity. So, a description of a beautiful woman might focus on her freckles or a crooked tooth or the way the muscles on her back move when she reaches up to change a lightbulb. I'm curious about doing the same thing in a script.
To the extent that it's something that's just costuming or makup, it seems like good sense to do the same sort of specific descriptions in scripts (maybe the pretty girl pulls her long sleeves down so that they hide her hands). But does it make sense to describe physical characteristics if they're mostly going to be a matter of casting?
Right. No, it does not. Unless a freckle is a story point, don't mention it. What if Rachel McAdams reads it, but doesn't have freckles, and now she doesn't want to do your script any more? That would suck.
If someone has a horrible wart on their nose, on the other hand, and they're self-conscious about it, then you would mention it because it's a character or story point. Like the way Dorothy is self-conscious because she's in black and white, and she dreams of being in color. Or something.
I will try to describe personality as efficiently as I can. I try to avoid describing looks for the lead actors. Rather than giving a character description that depends on casting, give a character description that depends on personality. Describe how the character is, rather than what they look like. Give the actor something to act, not something to have.
With minor characters, on the other hand, you may be looking for a specific type that will score with the audience immediately, e.g. WIRY BANK ROBBER, FAT CLERK, GENETICIST WITH BODACIOUS YA-YA'S. Then go for the physical description. But then, essentially, the character's type is the story point.