The text describes the use of parentheses for including additional information or enclosing asides (p. 149). On p. 151 em-dashes within a sentence are shown to describe “an abrupt change of thought.” The second em-dash example: “Everyone in the class—students and teachers—appreciated the joke” suggests that commas could be used instead because the students and teachers phrase explained the previous statement. Isn’t this including additional information, so technically couldn’t parentheses also be used?
Is there a standard such as using em-dashes within dialogue sentences and using parentheses within text? Is the choice of using parentheses/em-dashes/commas dictated by the editor’s style guide or is one option more preferable? I guess I'm just a little unsure about how the choice of specific punctuation can change the meaning or emphasis of a sentence.