Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
it and the dash (see the discussion on page 101). However, a colon
is usually most appropriate if the relationship between the opening
and concluding parts of the sentence is straightforward, and a dash
is appropriate if this relationship contains something unexpected.
If either mark would meet your purpose, it is preferable to use the
more low-keyed colon, as the dash is stronger and will lose its punch
if overused. Reserve it for situations where you want its dramatic
impact.
Caution: Some authors go wildly overboard with the dash, applying
it wherever any sort of break in a sentence seems to be needed.
Such sloppy usage may be acceptable in rough drafts and informal
memos, but will not enhance serious writing.
INDICATING INTERRUPTED DIALOGUE
In dialogue, the em dash serves to indicate broken-off speech. One
speaker can interrupt another:
158
"Listen, I'm serious," I said. "No kidding. Why's it better in the
East?"
"It's too involved to go into, for God's sake," old Luce said. "They
simply happen to regard sex as both a physical and a spiritual experi­
ence. If you think I'm-"
"So do I! So do I regard it as a wuddayacallit-a physical and
spiritual experience and all. I really do. But it depends on who the hell
I'm doing it with. If I'm doing it with someone I don't even-"
"Not so loud, for God's sake, Caulfield. If you can't manage to keep
your voice down, let's drop the whole-"
"All right, but listen," I said. I was getting excited and I was talking
a little too loud. Sometimes I talk a little loud when I get excited.
-J.D. SALINGER, The Catcher in the Rye
A speaker can stop abruptly without being interrupted:
". . . I took a corkscrew from the shelf:
I went to wake them up myself.
And when I found the door was locked,
I pulled and pushed and kicked and knocked.