Em dashes are handy little items for setting phrases apart for special attention, but be cautious when employing them, because when misused, they can obscure rather than assist in comprehension.
1. “For the most part, this water comes from aquifers—that’s groundwater—or from surface waters—that is, rivers and lakes.”
When em dashes come in pairs, what lies between is a parenthetical digression that merits a more dramatic break than that indicated by a brace of commas or two parentheses. If the parenthetical phrase ends the sentence, however, only a single em dash is needed.
Three or more em dashes in one sentence creates an ambivalence in the sentence structure. In this case, it’s better to use parentheses—and to avoid mixing em dashes and parentheses for digressions of equal or parallel impact, use them for the second digression as well: “For the most part, this water comes from aquifers (that’s groundwater) or from surface waters (that is, rivers and lakes).”
2. “Her recent roles have shown her interest—and her ability—to go beyond the usual popular movie.”