I know of two commonly used script formats for plays, the American and the British. By the way, if you're using a small monitor, this may or may not look right:
 
                                FRANK
In both the American and British styles, you use a Courier 12-point
font. In the American style, however, the character name is centred 
and in capital letters above the line of dialogue. Dialogue is single-
spaced and, unlike in screenwriting format, spans most of the page. 
Leave healthy margins though. Stage directions . . .
                                (Hitting the Enter key, then tabbing 
                                a bunch of times, he demonstrates.)
. . . are indented to the centre, single-spaced (Remembering.) If 
they're short, they can just be incorporated into the side of dialogue 
itself.
                                (FRANK stares at the screen, sips his 
                                whiskey.)
Character names are in caps in stage directions, too. But not, Dixie, 
when part of the dialogue itself.

                                DIXIE
What happens when the next character speaks? Or you want to indicate 
how a line is said?

                                FRANK
                                (Petulantly.)
Isn't it obvious?

Or: 

FRANK:        There's the British style, also widely used in Canada, 
              in which the character name goes in caps on the left 
              margin. Dialogue is indented about fifteen spaces, 
              single-spaced across the width of the page -- again 
              leaving healthy margins -- and with a justified, 
              meaning even, left margin. Stage directions . . .
                                (At risk of repeating himself, he once 
                                again demonstrates.)
              . . . are indented to the centre, even the short ones.
                                (Pause.) 
        
DIXIE:        And when the next character speaks? Or what about if
              they're doing something while they speak? 

FRANK:        (Smashing his whiskey glass against the wall.) How can 
              you ask me that?!

 
Of course, these days there's all sorts of scriptwriting software out there that'll do this work for you.

Why does format matter? Well, besides the fact that your script is liable to be taken a lot more seriously if it's prepared in a professional style, these formats create a clear, workable document for actors, directors, technicians, et al. to use in rehearsal.

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