Watson likes to take a back seat to other people (especially Holmes) when there's clue analysis to be done, but he also does his fair share of telling us point-blank what he thinks of Holmes's various clients and visitors. Take Mary Sutherland: "For all the preposterous hat and the vacuous face, there was something noble in the simple faith of our visitor which compelled our respect" (Identity.79). There's Watson engaging in direct characterization: he's not trying to suggest anything subtly. He's just telling us, straight out, that even though Sutherland looks kind of dumb, she still has finer qualities that both he and Holmes like.
Holmes makes a game out of seeing how much he can guess about a client from what he just sees of the person from his apartment window. And he's really great at it. But Watson's got a few skills, too. Check out this exchange about Mr. Jabez Wilson:
I did not gain very much, however, by my inspection. Our visitor bore every mark of being an average commonplace British tradesman, obese, pompous, and slow. He wore rather baggy grey shepherd's check trousers, a not over-clean black frock-coat, unbuttoned in the front, and a drab waistcoat with a heavy brassy Albert chain, and a square pierced bit of metal dangling down as an ornament. A frayed top-hat and a faded brown overcoat with a wrinkled velvet collar lay upon a chair beside him. Altogether, look as I would, there was nothing remarkable about the man save his blazing red head, and the expression of extreme chagrin and discontent upon his features.
Sherlock Holmes' quick eye took in my occupation, and he shook his head with a smile as he noticed my questioning glances. "Beyond the obvious facts that he has at some time done manual labour, that he takes snuff, that he is a Freemason, that he has been in China, and that he has done a considerable amount of writing lately, I can deduce nothing else" (League.14-5)
Watson observes Wilson's "not over-clean black frock coat," "faded brown overcoat," and "wrinkled velvet collar" and comes up with "average commonplace British tradesman." And he's not wrong. See, Watson's no fool. But Holmes – he's just epic. So he can look at the same clothing and see that "square pierced bit of metal" is actually a Freemason's compass. Underneath the wrist of that "black frock-coat" is a tattoo of a fish that indicates time in China, and his right sleeve is worn, hence, a "considerable amount of writing."
Using clothing as characterization – and as a point of competition between Holmes and Watson – is an economical way of telling us about the character being examined (Jabez Wilson). But it also emphasizes the Holmes/Watson dynamic: Watson's no slouch, but Holmes still leaves him in the dust, brain-wise.
Nearly all of the wrongs committed in these twelve stories are the result of family betrayal: "A Case of Identity" finds a stepfather and a mother conspiring against their daughter, "The Boscombe Valley Mystery" sees a father kill another man to stop him from marrying his son to the other's daughter, "The Copper Beeches" involves the forced imprisonment of a daughter by a father to extort money from her, and the list goes on and on. So we think it's fair to say that family life – and the decision to treat your family well or badly – is one surefire way to tell what kind of a person you are in a Sherlock Holmes story.
Conan Doyle often repeats that Holmes looks for cases based on their interest to him and not on the social status of the people involved. Indeed, Holmes often speaks favorably of working-class people: Mary Sutherland and Violet Hunter among them. Yet, he frequently disguises himself as a "loafer" ("The Beryl Coronet") or tries to mingle with "roughs" ("A Scandal in Bohemia"). Watson's assessment of Jabez Wilson, the pawnbroker, definitely depends on his occupation as a middle-class businessman, and "The Man With the Twisted Lip" is all about a guy with a really good education who's acting wrongly by begging for a living. And Watson characterizes governess Violet Hunter as someone with "the brisk manner of a woman who has had her own way to make in the world" (Beeches.17). Occupation, in other words, is definitely one way to suggest both social status and character.
One final method by which the Sherlock Holmes stories use some type of description to develop psychological depth is personal appearance. Take, for example, Dr. Grimesby Roylott, a man who beat his Indian butler to death over a burglary and who murdered his stepdaughter with a poisonous snake to keep her from getting married. Watson describes the man:
A large face, seared with a thousand wrinkles, burned yellow with the sun, and marked with every evil passion […] while his deep-set, bile-shot eyes, and his high, thin, fleshless nose, gave him somewhat the resemblance to a fierce old bird of prey (Band.95)
Roylott's vicious character and his willingness to harm people weaker who are than he is don't just emerge through observations of his behavior throughout "The Speckled Band." Those qualities are written on his very face, "seared" with wrinkles and "marked with every evil passion." Roylott's appearance is like its own textbook of evil, with its "resemblance to a fierce old bird of prey."