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 doesnít mention this option, but this is a common approach to communication when silence is important, for example in the usual military scenarios.
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.