You can't comment on Sheridan's art of characterization without mentioning the aptly named characters. (In fact, we even had a new word in the English language coined because of one of them!) It is this naming technique that distinguishes Sheridan's characterization art above many others.
Let's take a look at some of the more interesting of Sheridan's names in this book: Absolute, Languish, Malaprop, & O'Trigger. First, whether it be Captain Jack or Sir Anthony, the characters with the name of "Absolute" are very strong-willed! This fits their names well. The father is strong-willed in NOT having his son woo Lydia. The son is strong-willed enough to go against his father and WIN Lydia. How about this for being strong-willed:
Had I a thousand daughters, by Heaven! I'd as soon have them taught the black art as their alphabet!
Lydia, of course, is Lydia Languish. Perfect name for her in that she is forced to live in a less than ideal situation: so involved in fantasy that everyone seems below her. The characterization of Mrs. Malaprop actually helped us coin the term "malapropism" because of her ghastly use of the English language. And finally, the quick-to-act O'Trigger who is a cocky and overbearing Irishman remains true to his name as well (even in suggesting a duel he isn't ready for).
As you can see, then, it is the names of the characters in Sheridan's Rivals that mark his art in characterization.