OK, we know, that sounds like a copout: what the heck kind of a style is "variable"? But bear with us for a second. Here's our logic: Watson is certainly the central narrator of the Holmes stories, which means he has total control over the arrangement of who says what and when. But Holmes tales are heavy on dialogue and, even more importantly, they contain lengthy testimony from other characters, often inflected with the accent of those characters.
Let's take a look at "The Noble Bachelor," and the distinctive voice of Hatty Doran – or Mrs. Moulton, as she's known by this point:
The next I heard of Frank was that he was in Montana, and then he went prospecting in Arizona, and then I heard of him from New Mexico. After that came a long newspaper story about how a miners' camp had been attacked by Apache Indians, and there was my Frank's name among the killed. I fainted dead away, and I was very sick for months after (Bachelor.188)
Mrs. Moulton's slang – "I fainted dead away," "the next I heard" – is unlike that of any other character's. At the same time, the places she mentions are totally outside the experience of either Watson as the primary narrator or Holmes as the guy who talks the most in these stories. Let's compare her loose, emotional style with Holmes's analytical reasoning in the same tale:
From the first, two facts were very obvious to me: the one that the lady had been quite willing to undergo the wedding ceremony, the other that she had repented of it within a few minutes of returning home. Obviously something had occurred during the morning, then, to cause her to change her mind. What could that something be? She could not have spoken to anyone when she was out, for she had been in the company of the bridegroom (Bachelor.203)
Check out that "What could that something be?" in the middle of the passage there. Holmes is using this rhetorical question both to illustrate his thought processes and as a teaching tool, to instruct Watson (and the reader) in his methods. Holmes's voice appears within a page of Doran's and probably covers as much space in the explanation as her testimony does. But the style of the two passages could hardly be more different: Hatty gives an emotional confession, while Holmes offers a careful description of his reasoning. Hatty is supplying the missing clues that Holmes needs to wrap everything up for Watson and for the readers.
The contrast of these two passages shows us something interesting about the overall style of storytelling in The Adventures of Sherlock Holmes: Watson provides a frame for each story, an introductory series of paragraphs sketching some dialogue with Holmes or setting the scene of the investigation. His own style, as we discussed in "Tone," is often heavy on the imagery when he gets going. But he never talks for very long at one time. Watson almost immediately hands over the actual telling of the story to Holmes's clients, to Holmes's suspects, and to Holmes himself. If you look at any of the twelve tales, you'll find that most of the narration doesn't belong to Watson at all. Watson hates filling in background information. He'll even substitute newspaper copy rather than writing out his own version of events: see, for example, the "Hotel Cosmopolitan Jewel Robbery" article ("The Blue Carbuncle.61).
So why does The Adventures of Sherlock Holmes rely so heavily on first-hand testimony and distinctive, individual style? Well, we can't say for sure, but we think one reason might be the realism we mention in both Holmes's "Character Analysis" and in "Setting." We love Watson, but who are you going to trust: Hatty Doran's story, or Hatty Doran's story filtered through another person? Watson is an amazingly laidback narrator: he brings together all the bits of information you need to know, but he otherwise appears pretty passive about letting the story unfold in many different voices – at least, until Holmes is ready to bring it all home.