Usual correct practice is that a dash is used when speech is interrupted, and an ellipsis when it trails off into silence:
I grabbed him by the throat. 'Don't you dare talk to me like - '
'Shut up, you bastard!' He hit me across the mouth. |
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She said, 'I don't know if you think we should...'
I looked at her, but she didn't seem to know what to say next. |
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Dashes are properly used - as they were in the old days - in pairs, rather like parentheses (otherwise known as brackets).
On the other hand - well, it's a looser modern thing, isn't it? Using dashes for this kind of thing - the kind of hesitation in dialogue - the kind which isn't grammatical enough for a comma. I got endlessly picked up for doing this too much by my MPhil tutor. He was quite right. It's lazy, and too non-specific.
Emma