What themes are portrayed with Watson? How does Conan Doyle achieve this?

The dynamic between Sherlock Holmes and Watson places Holmes in the dominant position in their partnership; he is eccentric, brilliant, and so blunt in his dealings with others that his behavior can be off-putting. Because Watson is able to appreciate Holmes's superior qualities and overlook his shortcomings, Watson emerges as loyal, patient, understanding, and tolerant.
Watson's humanity enables him to interact with others in socially productive ways. He is able to bond quickly with Miss Morstan because he understands and values human feelings, such as fear, while Holmes seems to operate on intellect alone. Sir Arthur Conan Doyle furthers the theme of Watson's generosity of spirit through exchanges like this one that Watson narrates:

“You really are an automaton,—a calculating machine!” I cried. “There is something positively inhuman in you at times.” He smiled gently. “It is of the first importance,” he said, “not to allow your judgment to be biased by personal qualities."

Watson is unafraid to be candid with Holmes, which suggests a theme that partnerships can only thrive where there is honesty.