I will try to explain the general idea I've gotten from the most trusted source in my opinion.
PLZ EXCUSE MY CRUDE INSULT TO SCREENPLAY FORMAT, I DON'T KNOW HOW TO TRANSFER THE REAL THING INTO THIS APP.
The Ellipsis
Starting with the ellipsis, there were two main uses:
1) a pause in a speaker's single thought he is expressing. (Examples seemed like it was for dramatics.)
Joe
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I'm talking about a completely different school of thought ...
Joe scratches his chin.
Joe(Cont'd)
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... and it ain't the school of hard knocks.
2) the way I understood it was a pause in one's dialogue to think what or how to say.
Joe
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(Tears begin to form)
I'm not sure how to say this ... Dad was always the orator in the family.
3) Trail off.
Joe
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If I ever see your face up here again ...
M-Dashes
1) the interruption.
Joe
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I assumed he was just —
Bob
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I'm not going off some assumption, Joe.
2) the stammer
Joe
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(Aghast) I — I d — don't even know how that got there.
ANY HELP IS MUCH APPRECIATED🙂 ~ Matt