Acting for Two: The English dub has the same actress portray the Sea Witch and Jemmy the cat. This has often led to viewers believing the cat is the witch in disguise.
The Disney movie
Cut Song: Eric's "Her Voice", pieces of which can be heard throughout the score as sort of his theme. The musical puts it back in. "Silence is Golden" was an early song for Ursula, cut because the creators didn't like it. The musical has this trope in spades "Where I Belong" (Eric) and Ursula's reprise of "Her Voice" were cut after the tryout in Denver. 3 cut songs that still got to the public, via leaked demo tape are: "Wasting Away", "All Good Things Must End" and an alternate version of "Poor Unfortunate Souls (Reprise)". There are several the public has never heard, among them are: "The World Above", "That Oughta Show Her" and a reprise for it, and "Ursula's Incantation II".
Development Hell: Walt Disney himself commissioned an early storyboard that hedged much closer to the Andersen original, which the crew of the final version eventually discovered. You can see these sketches on the DVD.
Executive Meddling: Jeffrey Katzenberg caused the animators and others working on the film quite a bit of grief. One memorable instance was his insistence that the whole "Part of Your World" sequence be excised from the film, saying that he found it "boring" and also because he saw a child drop a box of popcorn and then struggle to clean it up during that moment in a test screening. Animator Glen Keane eventually persuaded Katzenberg to let them test the film again with a more adult audience and that scene intact, and the positive response convinced Katzenberg to leave it in. He admitted in a 2006 interview that the whole experience was "deeply embarrassing."
Fake Nationality: Samuel E. Wright, the voice of the Rastafarian crab Sebastian is actually from South Carolina.
Genius Bonus: Ursula's incantation for casting the spell on Ariel includes the words "Laryngitis" and "Glossitis". Someone well-versed in medical terminology would notice that they are a rather common conditions that may result in mute-ness.
Keep Circulating the Tapes: When it comes to the TV series, only about half of the series is available on VHS (plus three as part of the Disney Princess DVD line). The rest went unreleased.
The Other Darrin: Flounder has been replaced various times during the franchise, due to being (most of the time) voiced by a real kid. Jason Marin played him in the first film. Edan Gross played him for the first half of the television series, and was replaced with Bradley Pierce. Courtland Mead voiced him for merchandise released in the `90s (including the Animated Storybook CD-ROM, as well as a few toys), Cam Clarke voiced him in the sequel (due to him being an adult), and Parker Gloris played him in the prequel and the Kingdom Hearts series.
Buddy Hackett did not return as Scuttle for the TV series, so Maurice LaMarche filled in.
Chef Louie is René Auberjonois, years before becoming well-known as Odo on Star Trek: Deep Space Nine.
Mark Hamill has a very brief role at the very beginning as one of the sailors talking to Eric. While he was known for his work in Star Wars, this was a few years before he was well-known as an accomplished voice actor, with roles like the Joker, Fire Lord Oozai, or Skips.
Throw It In: Ursula's "Pathetic." from her Villain Song lyrics aside to Flotsam and Jetsam was actually ad-libbed by Howard Ashman while he was coaching Pat Carroll.
Troubled Production: Because of how labor intensive it would have been. Disney sent the animation of the bubbles to a Chinese-based facility, which just so happened to be located near Tiananmen Squarenote AKA the place where the Beijing student uprising occurred. Ultimately subverted in that the production went on as planned, but the labor was one of the reasons The Little Mermaid was the final Disney Animated Classic to have traditional cels and painting before CAPS from Pixar made things easier.
Unfinished Episode: There were plans for comics to expand on Ariel's mother. It was scrapped for being too dark. The character was later expanded on in Ariel's Beginning, the third movie, but vastly different. In the comic, dubbed Portrait of Life, her name was Atlanta and she befriended a human named Duncan. She dies saving Duncan from an avalanche Triton made. In the movie her name is Athena and she dies by being crushed by a ship, saving her oldest daughter.
Ursula was originally intended to be played by Divine, who died before he could record the role. Bea Arthur turned down the role because she was busy with The Golden Girls, and Jennifer Saunders auditioned but didn't get the role.
In addition, Melissa Fahn was originally supposed to play the role of Ariel, but was beat out by Jodi Benson.
A deleted draft detailing an alternate version of the ending had Ariel and Flounder trying to rush to the wedding barge only for Glut (the shark from earlier in the film who got stuck in an anchor ring) to ambush them, causing Flounder to gain a second wind, deliver an already exhausted Ariel to the ship, and bait Glut into chasing him with the barrel still lassoed onto his back (hint: it's actually a gunpowder barrel), causing Glut to bite the barrel and get caught in an explosion.
Broadway singer Elaine Stritch was the original voice of Ursula, but her constant clashing with the crew had her fired and replaced with Pat Carroll.
Ariel was imagined as a blonde early on, but the animators didn't want to be seen as ripping off Splash too closely.
During the production of Ariel's Beginning: a storyboard◊ from the film was presented. It appears that someone has Eric, Ariel and Sebastian hunting for a white whale and Sebastian isn't happy about it. There's also a woman in the storyboard who gets seasick like Grimsby in the beginning of the first film. It's implied that some talking parrot character will not get along with Sebastian and it's also stated that Ariel and Sebastian are "still angry with Eric". This was intended for another idea called "Arielís Undersea Adventures".
Ariel was originally going to lose her Seashell Bra after first being transformed into human, and sport Godiva Hair for the rest of the sequence. For whatever reasonnote Potential Fanservice issues, difficulty animating, budgetary constraints (Katzenberg was doing some severe editing with everything including schools of fish to keep the movie underbudget), this idea was abandoned.
Ariel's mother had a different backstory. One concept created by Peter David had her called Atlanta, and while she would've had some resemblance to Ariel, was going to have brown hair (young Triton had red hair, just like in the show). She befriended a human artist Duncan, and once he starts painting her, becomes famous. His rival Kole, jealous of his fame and discovering the mermaid, tries to kill him by pushing a boulder above him. But Atlanta performed a heroic sacrifice and pushed him out of the way. Triton kills Kole and spares Duncan, but takes his painting. After a week of solitude (that a 6 year old Ariel convinces him to get out of) his hair turned gray, and now has a hatred of humans.
Fun Fact: Jodi Benson, the original Ariel, and Sierra Boggess, who originated the role on Broadway, both graduated from Millikin University.
One of the models they used as a reference for Ariel's movements was Sherri Stoner, who would go on to voice Slappy Squirrel in Animaniacs. That's right: Slappy Squirrel was Ariel. Try wrapping your brain around THAT one...
This was the most successful film in the Disney Animated Canon since The Jungle Book, Walt's last movie, and proved to be such a hit, it convinced Disney to make it the first Disney Animated Classic to come to home video before going to the Vault (it was released on video under the Walt DisneyClassics brand, which was meant to be used for Walt's single-story animated films plus Robin Hood.)