As Watson gazes up at Holmes's room at Baker Street, he can immediately tell from his friend's body language that he is in the middle of solving a mystery. Through visual imagery and an idiom, Watson explains how he knows that Holmes is "at work again."
He was pacing the room swiftly, eagerly, with his head sunk upon his chest and his hands clasped behind him. To me, who knew his every mood and habit, his attitude and manner told their own story. He was at work again. He had risen out of his drug-created dreams and was hot upon the scent of some new problem.
In this passage, Doyle captures the appearance and movements of someone who is deep in thought. Holmes is not pensive in stillness, however, but rather paces around the room. This imagery contributes to his character development, as it shows him to be a restless and engrossed person.
Doyle, through Watson, also relies on an idiom for Holmes's character development. If someone is "hot upon the scent" of something, they are in pursuit of a clue or on the verge of solving a problem. This idiom, which signals an approaching discovery, is based on a canine metaphor. Dogs have a keen sense of smell, and often use this sense to pursue a track. Through the idiom, which is often used to describe investigators and detectives, Doyle implicitly compares Holmes to an eager dog. The imagery and idiom combine to introduce Holmes as a committed, restless detective.
When the King of Bohemia enters Holmes's apartment in the story's first part, Watson describes his appearance in detail. Combining rich visual imagery with allusion, Doyle develops the character as an imposing, yet also ostentatious and naive, member of the uppermost elite:
A man entered who could hardly have been less than six feet six inches in height, with the chest and limbs of a Hercules.
Hercules is a Roman mythological figure known for his immense strength and exceptional feats. Although many facets of his character are positive and inspire admiration, he is also emotional and violent. As a result of this, he is occasionally portrayed as naive and reckless. These ambiguous associations are present when Watson compares the newly arrived visitor to Hercules. His figure may appear tall and strong, but other elements of his appearance keep Holmes and Watson from taking him entirely seriously.
In particular, Watson focuses on the visitor's clothing, writing that in England, it would "be looked upon as akin to bad taste." Watson goes on to describe the elements of his outfit in detail, using rich imagery to develop the character as ridiculous.
Heavy bands of astrakhan were slashed across the sleeves and fronts of his double-breasted coat, while the deep blue cloak which was thrown over his shoulders was lined with flame-coloured silk and secured at the neck with a brooch which consisted of a single flaming beryl.
Whatever esteem the character's stature may have aroused in Holmes, Watson, or the reader is promptly dispelled by his garish clothing. With a description of the visitor's fur-trimmed boots, Watson concludes that this completes "the impression of barbaric opulence which was suggested by his whole appearance." To top it all off, he is wearing a broad-brimmed hat and a black vizard mask. His other clothing makes the presence of the mask ironic. Covering the top half of his face does very little when he is otherwise adorned in such over-the-top royal attire. The ludicrous attempt at disguise makes it easy for Holmes to conclude that, far from merely representing a royal house, his visitor is the prominent member of European royalty.
When Holmes reveals his visitor's identity, the allusion to Hercules becomes all the more apt. Hercules was the son of Jupiter, the Roman king of the gods. Although his great courage and exceptional strength come from the same place, Hercules's pitfalls are a result of his divine origins. He lives like a mortal but has the strength of a god, which makes it hard for him to regulate his powers. Similarly, it is safe to assume that the King of Bohemia has acquired his royal title through a patrilineal chain of succession. The identity of his parents has given him a life of opulence and influence, but it has also placed some limits on him. At multiple points in the story, for example, he regrets that his high station prevents him from maintaining a relationship with Irene Adler. It is because he is a king that he cannot afford to have the scandalous photo publicized.