Author Existence Failure: Poor, poor Mary Wickes, Laverne's voice actress. This was her final film role as she died of cancer during production and had to be replaced by a stand in, although thankfully only for a few lines. Fortunately, that stand-in is sound-alike Jane Withers, who took over the role in much-maligned DTV sequel.
Celebrity Voice Actor: Demi Moore at the time became the highest paid voice actor in Hollywood, receiving $1 million for lending her voice to Esmeralda. Also Kevin Kline as Phoebus and Jason Alexander as Hugo. The sequel casts Jennifer Love Hewitt as Madellaine.
Dawson Casting: Played with. Zephyr's voice actor, Haley Joel Osment, was nine when he recorded all of his lines, but when the movie came out, he was fourteen.
Disowned Adaptation: The descendants of Victor Hugo bashed Disney in an open letter to the Libération newspaper for their ancestor getting no mention on the advertisement posters for the film, and they harsly criticized the film itself as a vulgar commercialization of Victor Hugo's story.
Flip-Flop of God: The movie's directors have hinted that the gargoyles may all be figments of Quasimodo's imagination (a popular fan theory), but certain events in the film - namely Hugo startling Esmeralda's goat and them all taking part in the final battle - suggest they are real.
Irony as She Is Cast: Tony Jay, who was Jewish in real life, voices the evangelical Catholic Judge Frollo.
Mean Character, Nice Actor: In an interview for Disney Adventures that was published right around the time that the movie was released, Tony Jay made it a point to say that he was actually a very nice man in Real Life, despite continually playing villainous roles (and with Frollo being one of his darker ones).
Non-Singing Voice: Averted; with the exception of Esmeralda, every actor does their own singing. Tom Hulce, Quasimodo's voice actor, wanted to be a singer when he was younger.
Talking to Himself: During the flashback in the opening number, Jim Cummings voices about three or four characters in quick succession (one or two of the gypsy men, the boatman, and Frollo's guard). While Cummings does have an impressive vocal range, he unfortunately uses the same voice for all of these characters, making it very noticeable.
Technology Marches On: When the film was being promoted, the CGI people were touted as a huge technological advancement. They were the logical next step after the CGI stampede scene from The Lion King. Instead of using CGI to reproduce the same model making the same movements over and over again, the crowds of Hunchback were randomized, with different elements such as clothing, body types, and behaviors being mixed and matched to create these gigantic Ben Hur crowd scenes that would, otherwise, either be much too expensive to animate, or have to be static parts of the matte. It's more noticeable now (especially if you remember watching "Behind the Scenes" featurettes which showcased the various behaviors back on the '90s Disney Channel), but at the time, especially on a first-time viewing, they were pretty impressive and evocative towards the party atmosphere in "Topsy Turvy" especially.
Throw It In: Phoebus's actor named his horse "Achilles" for the sake of the "Achilles, heel" joke.
Phoebus's speech rallying citizens to rebel against Frollo during the climax of the movie was originally said by Clopin, but the directors/producers felt that Phoebus needed to be more involved in the uprising.
Instead of Victor, Hugo and Laverne the gargoyles would have been named Chaney, Laughton and Quinn as homage to actors Lon Chaney, Charles Laughton and Anthony Quinn who had played the title character in previous adaptations. This idea was brought back into the German musical only named Charles, Antoine, and Loni.
The prologue was originally going to be a straightforward narration. After a couple of read-throughs, the filmmakers found it too slow and plodding. So they rewrote it as a musical number - "The Bells of Notre Dame".
A much more tragic ending was considered. Frollo would have fatally stabbed Quasimodo before being killed by Esmeralda. Phoebus would have then found them and Quasimodo's last wish would have been to ring the bells one last time.
Fake Nationality: In the 1939 version, the French characters are played by British and American actors. In the 1956 version, most actors are French, but Esmeralda is played by the Italian Gina Lollobrigida and Quasimodo by the Mexican American Anthony Quinn.