Well aware of the war raging in Europe, Charles Laughton chose a lull in the day's shooting to recite, in full Quasimodo costume, Abraham Lincoln's Gettysburg Address, as he had done in Ruggles of Red Gap (1935). As in the previous film, it stunned the cast and crew for the rest of the shooting day.
RKO specifically wanted to outdo the 1923 silent version of the story, so a vigorous campaign that spared no expense was undertaken. Much attention was given to advance publicity; no pictures of Charles Laughton in full Quasimodo makeup and costume were allowed to be seen so that a first-time viewing would be a guaranteed shock. Also, the studio hired (at Laughton's request) leading makeup artist Perc Westmore to supervise makeup. Unfortunately, Westmore and Laughton had heated quarrels before a final image for Quasimodo was agreed upon.
This was RKO's last release for 1939 (and second costliest in its history, next to Gunga Din (1939)). Although it premiered about the same time as Gone with the Wind (1939), it held its own at the box office, grossing an impressive $3.155 million.
American Film Institute Catalog of Feature Films 1931-1939 includes Gail Patrick and Laura Hope Crews among the uncredited players, without role designations. Neither actress appears in the film in any role of prominence, which their status in the industry at that time would have dictated. It's possible, however, they participated anonymously as extras, just for the experience, as many of their contemporaries often did.
The trivia item below may give away important plot points.
Sound from King Kong (1933) is used in the film: when Esmeralda is being tortured, some of her screams we hear belong to Fay Wray. Also, when Quasimodo is defending the cathedral, some of the screams of the wounded attackers belong to the sailors from King Kong; and when Frollo falls to his death, his scream belongs to one of the sailors as well.