For some reason, I've spent the past few weeks in a Sherlock Holmes frame of mind. Maybe it's because I binge-watched a few Columbo episodes, or maybe it's because I read Nicholas Meyer's The Seven-Per-Cent Solution (1974) during my long stay in Pennsylvania and then revisited Michael Chabon's The Final Solution (2004). Meyer's novel explicitly positions itself in the Sherlock Holmes canon by claiming that the novel is an as-yet unpublished memoir penned by the steadfast Dr. John Watson, while Chabon's novel--never mentioning Holmes by name--draws some pretty obvious parallels to Sir Arthur Conan Doyle's inimitable consulting detective.
Now, where fan fiction gets dangerous is when it starts to flirt with actual historical events. Chabon's novel, which involves an elderly Holmes ripping through a mystery and reassembling its components, all the while criticizing the infirmity of his aged mind, stays strictly in the realm of the imaginary. Meyer's The Seven-Per-Cent Solution, however, claims to be a significant revision to the Holmes canon, as Meyer's rendition of Watson claims that Holmes's alleged death is a patent falsehood: Holmes vanished in order to kick his opium addiction.
And that's when Meyer commits himself to one of the greatest fanfic mistakes: introducing real-life, historical figures to the narrative. In this case, Dr. Sigmund Freud.
But Meyer's interpretation of Freud is surprisingly... human. (This differs drastically from a pretty awful made-for-TV movie I saw, starring Christopher Lee as Holmes and Patrick MacNee as Watson--they're in Vienna, where they meet a Sigmund Freud whose accent makes Joey Tribbiani's portrayal of Freud in an episode of Friends seem like an Oscar-worthy performance.) Freud, in Meyer's novel, comes across as a therapist, interested in his patient's recovery--not as a crackpotted stereotype interested only in pinning everyone's problems to childhood sexuality.
I still think, though, that Chabon's route is the safer of the two--and likewise results in the stronger book, as it doesn't cause the reader to pause and question the story. Chabon, who sticks to the particulars of scene and place, evokes reality in that manner, while Meyer skirts a dangerous line in attempting to use historical figures as characters, as opposed to compasses for indicating time and place.
(And if you want more on fan fiction, I defer to Strong Bad, who covers this topic in one of his scathing emails.)