Consider:
1. In many ways, Holmes is a Romantic rather than Realist. In the story’s opening lines, he muses that “life is infinitely stranger” than anything the human mind “could invent.” But he is soon presented with a puzzling set of circumstances, which prove not too strange for his mind to unravel.

2. Even before Miss Sutherland steps foot in his flat, Holmes uses his powers of observation to identify, even classify her. He makes other keen observations about his new client’s dress and appearance, deducing that she is shortsighted, does secretarial work, and has come to him in a rush.

3. As Miss Sutherland tells her tale, clues come in rapid succession (if you miss them, a second reading brings them to light): Miss Sutherland reveals that her mother and stepfather use her income; her mother seems to like Hosmer Angel more than Miss Sutherland does; Hosmer appears only when the stepfather is away on business; he has no address, avoids direct daylight, wears tinted glasses, and speaks in a husky voice—all indications of someone who wishes not to be recognized (because Miss Sutherland obviously knows him).

4. Watson serves as a literary “foil” for Holmes. A foil is a thin piece of metal on which is placed a gem (genuine or fake) for contrast, thus setting off the stone’s brilliance. A literary foil is a secondary character whose qualities offer a contrast with the main character’s traits. Poor Watson never gets it right, and he is slow to the point. His lack of acumen sets off Holmes’ astonishing brainpower.