As a writer, I always want to know where the light is in the room and how it’s striking the characters. Even if that description doesn’t make it to the end – maybe because the viewpoint character isn’t that observant – the echo of it there means that there’s a little bit more reality to the situation. Always knowing the basic blocking, like the stage directions, is very helpful in terms of creating specificity in your scenes.
Jeff VanderMeer shares a lecture from his writing guide Wonderbook: The Illustrated Guide to Creating Imaginative Fiction over on Electric Literature. Check it out for the fun illustrations; stay for the good advice.