Dashes, Semicolons, and Ellipses
I think you're confused about dashes, semicolons, and those periods (...), also known as ellipses.
Periods indicate a trailing off.
"I don't think that's a good idea..."
"Hush!"
Semicolons indicates a dramatic pause, which is later finished.
"This is not a good idea; it will lead us into danger."
"Hush!"
Dashes indicate an abrupt cutting off.
"I don't think --"
"Hush!"
What's the difference between the dash and the periods? With dashes, you cut off the sentence before it is complete. With periods, you use them to indicate a cliffhanger sentence. It's still complete grammatically, but the idea isn't.
This also should only be used in dialogue, unless you're writing in first person, which is a little less strict.