Develop backgrounds for each character and keep track of mannerisms and catch phrases for each.
At its heart, a screenplay is a blueprint for the film it will one day become. Wondering why Courier font is used? There are no car chases and special effects, so the people are what it is all about.
A screenplay can be an original piece, or based on a true story or previously written piece, like a novel, stage play or newspaper article.
That is another reason why the development of the characters is so important.
With roots in the playwriting genre, today, parentheticals are used very rarely, and only if absolutely necessary. Like so many other things, you need a talent for screenwriting and also lots of practice.