Fast, Wordy, Expressive
Plot details tend to pile up in mystery novels, and The Hound of the Baskervilles is no exception. There's a lot going on in this book, with new surprises every chapter, so the narrative often feels like it's racing ahead. Still, Watson also takes the time to describe things very thoroughly. Conan Doyle wants you to get a feel for each scene and setting, so that the mystery of the novel seems urgent and interesting to you as the reader. Let's take a look at a passage from one of Watson's telegrams to Holmes:
My previous letters and telegram have kept you pretty well up-to-date as to all that has occurred in this most God-forsaken corner of the world. The longer one stays here the more does the spirit of the moor sink into one's soul, its vastness, and also its grim charm. (8.2)
Get a look at those adjectives. "God-forsaken" is probably the most dramatic, but "grim charm" also gives us a sense of Dartmoor's barren beauty. Even Watson's choice of verb—that the spirit of the moor sinks into the soul—suggests the heaviness and darkness of the place. Watson uses a lot of words to tell us what we already sense from the plot of the novel: that Dartmoor is shadowy and somber. However, Watson's expressive descriptions add something of the flavor of his experiences as well as just the events.
There's a lot of dialogue in the book that sounds wordy and stilted to our modern ears. For example, Dr. Mortimer tells Holmes,
My motive for withholding [the Baskerville manuscript] from the coroner's inquiry is that a man of science shrinks from placing himself in the public position of seeming to endorse a popular superstition. (3.105)
He could have just said "I didn't want people to think I was, like, an idiot." Maybe people were just smarter back then.
But the language is probably not too unfamiliar if you've read other novels written at the time.