Because of the age difference, John Wayne felt very uncomfortable with the romantic scenes with Elizabeth Allen, and asked John Ford to cast Maureen O'Hara opposite him instead. Ford refused and replied "Are you going to be the one who tells her she's perfect because she's old?". After this movie, Wayne made the decision to confine his romantic interests to mature women.
The term "shanghai'd" dates back to the 1840s. Skippers of the China Clipper ships had trouble getting sailors to man the ships because most preferred to join the gold rush. Crooked bartenders would, for a price, slip a likely man a "Mickey Finn" (chloral hydrate) in his drink to knock him out. The man would wake up later "shanghai'd" and on his way to China as a deckhand.
In the beginning, Gilhooley tells the captain who "shanghai'd" him that they were supposed to be at the island "on the 7th". Taking into account the time-line of the movie, ending on Christmas, that would make Donovan and Gilhooley's birthday as December 7th, the anniversary of the Japanese attack on Pearl Harbor.
During the scene where Amelia is wishing to "charter the Araner", Donovan states that he owns the Araner and a boat named the Inisfree. Inisfree is the name of the village in Ireland in which The Quiet Man (1952), another John Wayne / John Ford film, takes place. Araner is the name of John Ford's personal yacht, which is the actual ship used in this film.
A few years later John Wayne suggested Lee Marvin should make another film with him. However Marvin had become a major star after Cat Ballou (1965) and told Wayne that he was no longer interested in playing supporting roles. Wayne could never really believe that Marvin was being paid $1 million per film in the late 1960s.
John Wayne's character's full name is Michael Patrick Donovan. However, he is rarely referred to by his given name. He is called either "Guns" or, occasionally, "Donovan" by most people throughout the movie. His full name is used only once, by Amelia when reading from the monument. He is called "Mike" several times, including the Captain of the Araner, LaFleur and Andre. As "Michael" by Sister Angelique (in French), Sister Mary Margaret, Andre and by Amelia on several occasions. Most notably Amelia, during their last scene, where she addresses him as "Michael" three times and "Michael Donovan" once.
While the film is set on the fictional island of Haleakaloha, which has a French governor, the only Polynesian language exhibited in the film is Hawaiian - "Haleakaloha" can be translated as "Home of Laughter and Love" (hale = home, aka = laugh, aloha = love) - and Amelia has come from Boston, MA by sailing ship.
In a bit of tongue-in-cheek, portraits of the founder and leaders of the Dedham Shipping Company are all of Jack Warden (in appropriate period dress). The portrait of Manulani appears to be a similar treatment of Elizabeth Allen, as she would appear if she were Polynesian or hapa.
John Wayne and Lee Marvin would often go out drinking together in Hawaii, despite John Ford's efforts to keep them apart. One night, Wayne returned to his hotel after a drinking session, when he saw a group of Catholic priests and launched into a vitriolic attack on the Catholic Church. Guilt kicked in almost immediately and Wayne made a donation to a church charity on the spot.
A mistaken use of a blasphemy slipped past the censors. At the end of the first fight between John Wayne and Lee Marvin, after Jack Warden has broken it up, Wayne and Marvin start to get out of the pond, but Marvin slipped and fell back into the water. As he did so, he exclaimed, "Jesus!" Because it was funny, the error was left in the film, and because the sound of the water partially obscured the exclamation, the dialogue was also left unaltered.