The ninth commandment of Detective fiction is:
The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.
In his 1939 commentary on his decalogue, Fr. Knox said:
This is a rule of perfection; it is not of the esse of the detective story to have a Watson at all. But if he does exist, he exists for the purpose of letting the reader have a sparring partner, as it were, against whom he can pit his brains. ‘I may have been a fool,’ he says to himself as he puts the book down, ‘but at least I wasn’t such a doddering fool as poor old Watson.’
This is an interesting commandment because, as Fr. Knox notes in his commentary, a Watson is entirely optional. Plenty of good detective stories have no Watson. In fact, thinking over my favorite detective series, the only one which has a Watson is Sherlock Holmes—that is, the only Watson in my favorite detective stories is the original.
Occasionally Poirot had Captain Hastings, but he’s much rarer in the actual Poirot stories than he is in the David Suchet TV series. In the Lord Peter Wimsey stories Charles Parker was more of a co-detective than a Watson and Harriet Vane certainly was a co-detective. Hugh Berringar was a co-detective with Cadfael. Jessica Fletcher usually didn’t have anyone investigating with her and the gang in Scooby Doo was a team.
Interestingly, I’ve also read all but one of Fr. Knox’s Miles Bredon mysteries and there is no Watson character in those, either. His wife is sometimes his foil, but she is generally a co-detective, using very complementary skills to his.
As something of an aside, but also somewhat on point, police characters who occupy an in-between state as a sort-of Watson and a sort-of co-detective don’t seem to last. I’m basing this on an admittedly small sample size, but in Lord Peter Wimsey Charles Parker was a major character in the first two books, a fairly prominent character in the third, then progressively dwindled in significance until he becomes just a minor footnote in the last few (he married Lord Peter’s sister before his slide into irrelevance).
In the Miles Bredon stories, Inspector Leyland is a major character in the first two novels, then a mostly ancillary character in the third, and absent entirely from the fourth and fifth novels.
By contrast, the at first under-sheriff and later sheriff Hugh Beringar is absent from only a few Cadfael stories—The Summer of the Danes and Brother Cadfael’s Penance come to mind—which are, admittedly, later on, but The Holy Thief is between them and Hugh is a significant character in it.
It’s interesting to contrast the character of Hugh Beringar with Charles Parker and Inspector Leyland because it gets somewhat to the problems with a partial-Watson. By contrast to the other two, Hugh Beringar was intelligent and quick-witted. A scene which particularly stands out in my memory was from Saint Peter’s Faire, where after telling Cadfael that he too had deduced something Cadfael did, he said, “I may not pick up on all the subtleties but since knowing you’ve I’ve had to keep my wits about me” (or words to that effect). Since he was intelligent he was allowed to have a personality.
Charles Parker and Inspector Leyland, by contrast, partially serving the function of a Watson, couldn’t really have much in the way of personality. An everyman simply can’t be very distinctive or he ceases to be an everyman. It’s not, of course, strictly true that Charles Parker had no personality—we did learn that he read theology in his off hours to relax from his official duties. But we never found out that he learned anything from it; this passtime never informed anything he said.
Leyland didn’t even have any hobbies that I can recall reading about.
What makes these police inspectors different from Watson was, I think, the nature of their attachment to the detective—happenstance. Watson, by contrast, was attached to Holmes by friendship. Oh, granted, Charles Parker was in theory a friend of Wimsey, but we never saw any of it and Wimsey wasn’t really the sort of man to have friends. Holmes and Watson, by contrast, really loved each other and were comrades. Watson accompanied Holmes purely because he was devoted to him and Holmes brought him because Watson was his friend.
Cadfael and Hugh form an interesting comparison to both; Hugh was an officer of the law but also a close friend of Cadfael. In fact, Hugh and Cadfael were close enough that Cadfael was godfather to Hugh’s first son. Even when Hugh had no part in an investigation he might show up to spend Cadfael merely for the pleasure of company. And therein we see what’s necessary for a police friend to stay a character—his office must be his secondary connection to the detective, even if it was his original connection.
It is not viable, long-term, to have the same police inspector working with the same detective on every case. (Though I will grant that Monk made it work to some degree, since Monk was a consultant. Ditto for Sean Spencer in Psyche. That said, police consultants come with their own problems since they need to operate under police rules, and there’s an inherent tension with the police constantly hiring someone to do their job for them. Psych got around this by being a comedy and playing this tension for laughs.)
So coming back to the Watson in a story—I think that Fr. Knox is mostly correct, but a true Watson is the exception rather than the rule. It is common for detectives to not act entirely on their own—it is not good for man to be alone—but co-detectives are far more common and I think generally a better choice. And co-detectives should be intelligent; they are characters on their own, but they are also somewhat of a stand-in for the reader helping the detective and who would prefer to think himself incompetent?
Either way, it works much better for the detective and his associates to have a genuine affection for each other.