This is by no means a "rule," but using different ellipses marks can denote different things.
(despite what is commonly stated, both ... and - are ellipsis marks. Hell, even a * can be an ellipsis. But, most people mean ... when saying ellipsis)
Anyway, ... is used to either show a pause in dialogue or the omission of a word(s), such as:
TOM
But, I...can we
just go?
It can also be used at the end of a sentence to indicate that the dialogue trails off/fades out.
TOM
But, I...
A tear runs down his cheek.
MARCY
Come on. Let's get
out of here.
The dash/dashes, - or -- , are used to indicate that someone has been cut off by the next person, as in:
TOM
But, I-
MARCY
Look out!
Someone else can chime in about using beats.
HTH
