I regularly am asked, "How detailed should I make my stage directions?" While the rules are never set in stone, as a playwright, it's important to understand the difference between action, business and blocking. My philosophy can be summed up in one sentence: Always write action, sometimes write business, never write blocking.
Action is what is essential to the scene, what causes a change in the circumstances on stage and/or in the play. It could be a character exiting, drinking from a poison cup or as small as kissing another character on the cheek. Business includes things that characters do during the scene that help develop them and the scene but don't necessarily cause a change in the play. Blocking is a description of where the characters move. For example, in The Locker Next to Mine, one of my favorite scenes is between Chuck, something of an outcast though he tries to hang with the cool kids, and Dalton, a popular lacrosse player. Chuck has just done something awful to another kid at school because he thought one of the popular girls would like him--and maybe even go out with him--if he did it. Of course it didn't work out that way, and he's destroyed. The scene is set in a locker room. I wrote no blocking, as I trust the director and the actors to figure that out, but Dalton has business: he changes for a lacrosse game. Sure, he could be doing something else, but this helps develop who he is. In the meantime, he discovers that Chuck is crying, which is action in that it causes a change in the circumstances on stage, and while Dalton's response isn't a physical action, it is a dramatic one: he extracts a promise from Chuck that "you won't do something stupid."