What Does a Play Look Like?
Not Like This!
You may have seen plays that look something like this:
ALEX. I want somewhere with a lawn. MERC. What? That patch of dead grass on 133rd not good enough? (Merc eyes the lock on the box of women's clothing.) Wish I had a lawn. I would've been a different person. (Beat.) Make sure you get a lawn. (Beat.) You been through your Mom's clothing? ALEX. (Lying) No. You?
Notice that character name, dialogue and intermittent (stage directions) extend from left margin to right margin, except for a small indent of the first line. Text is single-spaced.
This is Published Play Format, typically what the publisher gives you in an Acting Edition, and its goal is to save space. It's hard to read, and not submission format. Submitting a script in this format is a bad idea - it would surely give a theater's overworked literary staff a headache.
What Should My Play Look Like?
Playwrights and the people who read their work have never adopted an ironclad, industry-wide format, maybe because theater, by its nature, tends toward the revolutionary and can't bear to become establishment. Maybe we're just not that organized.
But even if there's not one, absolutely must-follow format, there are definitely common-sense formatting principles of "readability" that must be respected. If a work is going to be read by potentially many people you must place the words on the page in the most familiar manner. This will assure the reader that an experienced writer is behind the work and that same writer will not burden the reader with unusual markings, fonts, or margins. Here is a general rendering of Manuscript Format.
Note: Script formatting software has made formatting all scripts considerably easier and less time-intensive in recent years, and many of these same programs have playwriting templates, with settings that you can modify with relative ease. They're definitely worth investing in and more on those later.