'Getting people on and off stage from that living room set was a challenge. You can't always have the phone ringing.'-- Wendy Wasserstein
There are a few playwrights still around who imagine the entire set down to the geranium in the corner before they start writing. If you think in a highly visual way, go to it. But doing this isn't necessary -- or even very helpful -- before you start writing.
What you do need is an idea of the visual feeling of the setting. Sensing that will help drive the Subtext of your characters. And it will give you a visual benchmark for the tone of your play.
The best Rule: Lay out what you feel is absolutely essential for you to begin putting words on paper. Don't do more than that. A growing number of playwrights use the David Mamet approach where you figure you'll deal with all this when you go into production. Here's all he indicated for
The play takes place in John's office.
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