Two very different current novels of Sherlock Holmes have been filling the cracks in my reading time of late, and the natural comparison of the two has brought me to a couple insights on the dangers of writing pastiche.
Why do most non-professionals write their own Sherlock Holmes stories?
Well, obviously, because they like Sherlock Holmes. They like Sherlock Holmes so much that they want to write a story about him. And they know Sherlock very well.
Wherein lies the problem. If you like Sherlock Holmes as much as the average fan, you're very apt to center your story around him . . . to the detriment of the other characters in the piece. And since both you and a lot of your readers know Holmes so well, there is also a tendency not to flesh him out as one would a totally original character. So if the other characters aren't being written fully formed as Sherlock takes center stage, and even Sherlock is getting shorthanded . . . well, there aren't going to be too many truly deep and interesting folk in the story.
We have to care about the other people in the story to give Sherlock something to do that matters. Conan Doyle had a great hand at populating London with colorful, memorable folk, and their tales were what made Sherlock famous as much as the detective himself.
Of course, there are those writers who like something else as much as Sherlock Holmes and wind up using Holmes and Watson as framing devices to write about their other passion. A classic example of that sort of bad pastiche was Sherlock Holmes in Dallas, a novel in which Holmes and Watson came forward in time to try to solve the Kennedy assassination, only to be mired in historical detail without actually interacting with it. They wind up as mouthpieces for the author to regurgitate facts that one could read in a Kennedy assassination book without them.
Both books I'm reading have moments that might have been quite inspired had the writers built just a bit more fictional reality around them. But the fact that I'm handily switching back and forth between the two says something about their ability to draw me in.
It's a problem that goes back nearly as far as Sherlock Holmes himself.