There's a "common knowledge" floating around that one can deduce the approximate generation of a screenwriter based on whether they use one or two spaces after a period: Older writers who learned to type on manual typewriters usually add two spaces, and younger writers who learned on a word processors with proportional fonts use only one space and let the software manage the spacing width.
This week the Washington Post had an article> discussing the classic, controversial 1-vs-2 spaces controversy. Suprisingly (at least to me), of 60 college students recruited for the study, 21 were "two-spacers". But there's more. From the article:
The researchers then clamped each student's head into place, and used an Eyelink 1000 to record where they looked as they silently read 20 paragraphs. The paragraphs were written in various styles: one-spaced, two-spaced, and strange combinations like two spaces after commas, but only one after periods. And vice versa, too.
And the verdict was: two spaces after the period is better.
Standard screenplay format for printed script pages uses a monospaced font such as Courier, Courier New, Courier Final Draft, or Courier Prime. These fonts are designed to replicate the look of old-time manual typewriters like the Smith Corona Courier series.
Whatever side of the proportional-typeface debate you may be on, there is now a pretty compelling study that for readability, two spaces are best for writing screenplays.
With that said, some rules-- especially in the arts-- are made to be broken. Anyone try using three?