More on style sheets
You might want to check out this site, the VCU "Playwriting Seminars" site, which is probably the most influential one in the U.S.
http://www.vcu.edu/arts/playwriting/seminar.html
I also recommend the BBC site that someone linked to in a post above, for obtaining a template. A couple of caveats: Use the stageplay template for the U.S. (not the U.K.) if you are writing for the U.S. And you should be fairly competent in using Microsoft Word, or you will not be able to tweak things when needed, and you may be terrified by the plethora of paragraph styles.
As for Samuel French, I am tired of hearing about its "style sheet," since I have never been able to find it referenced anywhere on the
http://www.samuelfrench.com website, although many other sites tell you to "call" Samuel French to purchase the style sheet. The Samuel French website itself, in its information about submitting plays to Samuel French, just says to use "standard format," but makes no reference to its perhaps mythical style sheet. Many other contests say to use the Samuel French style sheet or copy the format recommended on the VCU site.
I have spent several years looking at various "style sheets" on the Internet. I even bought one, which is almost identical to what you will find on the VCU website for free.
The fact of the matter is that there is no ONE style that is absolutely the standard for stageplays. The VCU style is a good representation of a theoretical standard, though it varies from some other styles in a few particulars, such as how to handle scene numbering and page numbering and what font to use. (Most contests specify a "readable" standard font; some style sheets specify Courier.)
Some styles use a blank line after character name and after a parenthetical, though dialogue itself is single-spaced (usually).
If you use a style with a lot of white space, you may approximate the SCREENPLAY standard of one minute per page, but in general a STAGEPLAY will move more slowly, and a page will run somewhere between a minute to a minute and a half (at least).
Remember that dialogue runs from left page margin to right page margin. A long speech in a play can be really, really long, and it is not necessarily rattled off at a fast rate. A page can run a long time.