* His research indicates, surprisingly, that caffeine has healthy qualities.
* He tells his story chronologically, except the account of his accident.
* A severe frost, quite early for August, wiped out the tomato crop.
Here is where the intention of the writer makes the difference. In the examples above, I could have substituted dashes if I wanted to stress the interruptive elements. I decided that I didn't want to put the emphasis on the parenthetical expressions. The first sentence could have read: His research indicates—surprisingly—that caffeine has healthy qualities.
Parenthetical expressions, set off by commas,
are "asides," and add interesting-but-not vital information.