Plays are meant to be seen on stage, not on the page—right? Right, but before it makes it to the stage, the play must make it through a reader or a small army of readers. I've been one. And then, of course, assuming you survive the readers (and the literary manager and the artistic director), there is the director, the actors, the designers, etc. Each of them wants as clear a picture of your play on stage as possible. A few thoughts on how you can help them:

Keep your stage directions as compact as possible, with active verbs. In other words, instead of "Jennifer is sitting on the carpet" at the opening of a play, "Jennifer sits on a carpet." Note that I didn't write "Jennifer sits down."

Introduce each character with a one-liner which tells us his age and gives the reader (and potential producers) a handle on her: "Annie, mid-20s and a walking accident." Remember, if I'm a producer, I want to know who I can cast in the role; I want to know the character's age and type.

Avoid "overcreating" a character (even if you've come up with this information in your police file—see Building Characters above—keep it to yourself): "five-foot-three, with red hair and green eyes and heavyset." Unless it's crucial to the play, you're wasting time on something that's irrelevant. What if there's a really good actress who's five-foot-five or has brown hair? You'll come off as an amateur.

In this same vein, don't direct the play on the page. Avoid "line readings"—don't preface every line of dialogue with "sadly," "angrily," etc. I only use line readings like these if it's crucial, which means I might have a handful in a full-length play. Instead, put in a "pause" or a "beat" and leave it to the actors and director to figure it out. You might discover something interesting.

Stage directions are written to be read. Make them well-written and clear. There's nothing wrong with giving them a little spin, but don't write a novel. A teacher of mine, when talking about screenwriting, told me never to clump more than six lines of description together. In playwriting it's OK to break that law, but it's not a bad thing to keep the spirit of it in the back of your mind.