The story has a theme seen in some other Conrad stories (Lord Jim, Nostromo) of the hero facing a decision and drifting into the one which is "wrong" by normal social standards, a lapse from the rules he is expected to follow. The decision by the captain to conceal Leggatt is the hinge point of this story. While the reader is likely to believe that Leggatt does not deserve to be convicted of murder, there is no doubt that the captain, as the representative of the law on his ship, was breaking all the rules in concealing Leggatt. For the sake of a man whom he had known for only 15 minutes, and after hearing only this man's version of events, the captain takes an enormous risk of ruining his own career and reputation.
Yet interestingly, there is no clear point at which the decision is made. Although our narrator is speaking his thoughts, fears and doubts, he never says "I decided to conceal Leggatt". When they move from the deck to the cabin, it seems at first only to get Leggatt dry and to hear his story more fully, but the situation somehow drifts into one of concealment. This happens when they instinctively lower their voices on hearing the footsteps of the second mate (who has now taken over the watch) on the deck overhead. "I, too, spoke under my breath" the captain says - from that point the conspiracy is in place.
Despite the captain's fears, it is clear that no-one else in the ship has any suspicion of the secret even to the end. It is even possible to see Leggatt as entirely a figment of the captain's imagination, a doppelgänger, or a ghost - perhaps Leggatt actually drowned and sank after reaching our ship's side. At the end of the story, when Leggatt drops into the sea from a stern porthole, we do not even know if he survives the long swim to the Indo-China shore, to start a new life.
The story was created in just 2 weeks, while Conrad was writing Under Western Eyes. He wrote "The Secret Sharer" as a break from his much larger novel that was emotionally difficult for him. There are similarities between the two stories, with the Captain and Leggatt becoming Razumov and Haldin respectively. The story originally appeared in Harper's Magazine, under the title "The Secret-Sharer", but Conrad revised the title to make it more ambiguous, making Leggatt secretly share with the captain, rather than merely sharing a secret.
The story contains elements of real events – the chief mate of the Cutty Sark killed another crew-member for insolence during a storm, and was later arrested in London for his murder. Conrad also drew on his own time as captain of the Otago, when his first mate did not trust him, and got a particular scare when Conrad maneuvered the ship dangerously close to rocks in the Gulf of Siam.
The 2014 film adaptation received good reviews, while taking liberty with the story (most notably, Leggatt's character is transformed into a Chinese woman). It was praised by the seafarers' charity Apostleship of the Sea for its portrayal of some of the issues facing seafarers.