In what way is Watson a character foil for Sherlock Holmes? Technically speaking, does he have any other role or function?
Sherlock Holmes to John Watson:
"It may be that you are not yourself luminous, but you are a conductor of light."
-- The Hound of the Baskervilles
John Watson is often a misunderstood character. He is not a dim-wit at all. He is quite an intelligent man and a very brave and reliable person. He is not only the roommate and companion of Sherlock Holmes but a dear friend whom he has often relied upon in matters of life and death. And if one reads through all of Doyle's stories, it is clear that Watson himself has learned the methods of deductive reasoning that Holmes has been a strong proponent of. Holmes may be more intelligent and intuitive than Watson, but there is no doubt that John Watson is a smart man as well.
Sherlock Holmes often relies on Watson as a sort of mirror to reflect and consider ideas and theories when he is faced with a mystery. He is able to make inferences through his discussions with Watson that he may not have reached alone. In more than one case Watson becomes vital in helping Holmes understand the true nature of an incident that they are investigating.
Another important point is that Holmes relies constantly on the courage of Watson. He often tells Watson to bring his gun with him and knows that he can always rely on Watson no matter how dangerous the situation may be. Watson is Holmes's companion and comrade.
Watson, as a character, allows the deductive genius of Holmes to shine through. By asking questions that are seemingly obvious to the master detective, Watson acts much like Holmes's personal promoter, allowing him to display his logic and reason exponentially.
Watson's personality, being quirky, occasionally nervous, and perhaps a little dim-witted in comparison to his contemporary, is in direct contrast to the dry, extremely precise characterization of Holmes. In many ways, the men are opposites, which may lead one to believe that Watson is indeed the foil of Holmes. Yet, if we examine closer, we see that Holmes as a protagonist would be utterly lifeless were it not for the dialogue and opportunity provided by Watson. If anything, the men serve as co-protagonists in the stories, with Watson serving as narrator and assistant to the great detective.