A person’s actions in The Three Musketeers typically indicate his rank or his moral fiber. When Athos refuses to bargain with the horse dealer, for instance, and when he refuses to accept the money of the man he slew, this indicates that he is an aristocrat of very high birth. On the opposite side of Athos is Monsieur Bonacieux, who, when taken prisoner, exhibits very low moral fiber as he wails and complains and faints.
This is tool of characterization is especially used to delineate the differences between Athos and Porthos. While Porthos likes to deck himself and his lackey out in the latest and most ostentatious fashions, in an effort to make himself appear to be a man of true quality, Athos wears very simple garments but has the correct and true demeanor of an aristocrat. The richness of a character’s clothing in The Three Musketeers, therefore, may belie his or her true character.
Of the Duke of Buckingham, we learn: "One of the salient points of his character was the search for adventures and a love of romance." (12.3) That’s definitely telling, and not showing. The narrator will typically explicitly state aspects of characters’ personalities, which is very helpful since we then don’t have to spend time deducing them on our own.
In The Three Musketeers, physical appearances tend to dictate a person’s character. Athos looks like a nobleman, the mercer has the appearance of a tradesman, and D’Artagnan has the nose of a Gascon, which carries with it a whole host of attributes such as pride and poverty. If a person looks trustworthy in this novel, they typically are trustworthy. The one exception to this rule is Milady, whose manipulative powers are so great that, for example, a man will look at her and believe she’s a virgin, even when she’s clearly not.