Fun with dialogue and the em dash (—) as it interrupts continued speech with some kind of action. This specific tip holds my interest as I’ve been trying to settle the question in my own writing. As with many grammar tips, the finer points are often subject to personal style.
Most are familiar with the use of the em dash for interrupted speech:
Jan said, “Don’t you know you shouldn’t mix ammonia and—“
“I know what I’m doing,” Marvin snapped. “I’ve done it lots—“
Overcome by fumes, Marvin slumped to the floor.
But what happens when the speech is interrupted in the middle of a sentence and then continued? Here’s what the Chicago Manual of Style has to say about it:
If the break belongs to the surrounding sentence rather than to the quoted material, the em dashes must appear outside the quotation marks.
“Someday he’s going to hit one of those long shots, and”—his voice turned huffy—”I won’t be there to see it.” (taken from 6.90)
In our situation, that would look something like this:
“Don’t you know you should mix ammonia and”—Sal stepped back as Marvin slumped to the floor—”bleach.”
Note, however, that if the speech is interrupted with a speech tag (said, muttered, snapped), then you need a comma after the em dash and before the closing quotation marks:
“Don’ you know you shouldn’t mix ammonia and—,” Sal started to argue, but Marvin slumped to the floor.
So with simply interrupted speech, the dash goes inside the quotation marks, but interrupted then continued speech has them outside the quotation marks.
Ah, but wait.
While delving into the use of interrupting/continuing em dash, I came across this little tidbit on the Punctuation Made Simple website:
 An divided quote (interrupted by something narrative rather than attributive)
“My dear sweet friend”—sarcasm oozed between his words—“I’m so sorry.”
“I’ll take that one”—she pointed—“and that one, and that one too.” (She is pointing while speaking, but the words were spoken continuously)
[9A] If the actual quote is interrupted, the em dash goes inside the quotes
“Because—” Linda rubbed her chin “—I just don’t know what to do.” (The spoken words are pause during the action.)
That’s a new one for me. Has anyone else seen the punctuation differentiated this way? I’ll have to keep an eye out for sandwiching em dashes inside the quotation marks.