Here’s something I used to think about, back in the before-times: A clause set off by em dashes is like dropping underwater while swimming breaststroke—just a quick dip before popping back to the sentence’s surface. A parenthetical clause is more like diving down to the pool bottom to pick up a coin. And a footnote is a full-blown scuba dive—you have strapped on equipment and left the surface behind and you had better, after going to all that trouble, see something interesting down there.
Ben Dolnick