Who is Dr. Watson?

Dr. Watson is indispensable to the Sherlock Holmes stories. First of all, he is the narrator in almost all of them. The reader sees everything from his point of view. Secondly, he is an active participant in Holmes' cases and adventures. In "The Hound of the Baskervilles" Watson has an even more important role than Sherlock Holmes because Holmes sends him on ahead to Baskerville Hall where he sees the mansion, the moor, and meets all the other characters first.
In addition to being the narrator and the great detective's loyal companion, Watson serves as an interlocutor, a word defined as "a person who takes part in a dialogue or conversation." Through this literary device the reader is able to learn something of what is going on in Holmes' mind. A good example of both Watson as loyal comrade and as interlocutor can be found in "The Adventure of the Speckled Band" when the two men arrive at the Crown Inn near Stoke Moran and are waiting for Helen Stoner to signal them with a lamp in her window.
"Do you know, Watson," said Holmes as we sat together in the gathering darkness, "I have really some scruples as to taking you to-night. There is a distinct element of danger."
"Can I be of assistance?"
"Your presence might be invaluable."
"Then I shall certainly come."
"It is very kind of you."
"You speak of danger. You have evidently seen more in these rooms than was visible to me."
[This is where Watson becomes the valuable interlocutor.]
"No, but I fancy that I may have deduced a little more. I imagine that you saw all that I did."
"I saw nothing remarkable save the bell-rope, and what purpose that could answer I confess is more than I can imagine."
"You saw the ventilator, too?"
"Yes, but I do not think that it is such a very unusual thing to have a small opening between two rooms. It was so small that a rat could hardly pass through."
"I knew that we should find a ventilator before ever we came to Stoke Moran."
This conversation continues, with Holmes calling attention to various aspects of the room they both examined earlier. Finally:
"Holmes," I cried. ""I seem to see dimly what you are hinting at. We are only just in time to prevent some subtle and horrible crime."
Dr. Watson's three functions as narrator, comrade, and interlocutor are to be found in most of the Sherlock Holmes stories. He is truly indispensable.