The Ultimate Screenplay Format Reference

Mute Dialog

A character may be mute and communicate using sign language. As the general public is not familiar with sign language the meaning must be communicated somehow - either orally or by subtitles.

The character - even though mute - could speak while he signs:
 

                    
                     MUTE PERSON
               (while signing)
         Do you understand me?
 

If the dialog is written in subtitles, this should be mentioned in the parenthesis:
 

                    
                     MUTE PERSON
               (while signing; in subtitles)
         Do you understand me?
 

If the mute character is a major character, it might suffice to mention his/her muteness once in the narrative instead of every time he/she speaks.

Another option would be to just have the character sign without speaking, in the hope that the audience will get the meaning. Trottier doesn’t mention this option, but this is a common approach to communication when silence is important, for example in the usual military scenarios.

Dialog Punctuation

--

The double dash indicates interruption: a sudden shift or break in thought, for example when one character interrupts another, a character is interrupted by an event (sound, action), the character interrupts himself by shifting his thought or he speaks interrupted. Trottier also mentions using the -- when the character speaks with sudden emphasis.
 

                     VIVIAN
         I’m sorry you had to wait --

                     JIM
         I’m tired of your excuses.
 

...

The ellipsis is used to indicate continuity. A character starts speaking, pauses and continues to speak. Also when a character finishes another character’s sentence the ellipsis is used.
 

                     VIVIAN
         On second thought ... I’m not
         that sorry.
 

(beat)