When Scar says "You have no idea," in response to Simba telling him he's weird, it's said with the exact same intonation as when Claus von Bülow says it in Reversal Of Fortune. Both characters were played by Jeremy Irons.
All-Star Cast: The film that started the trend of hiring a slew of name actors for an animated film rather than less well known voiceover specialists. Though it is not the first to do so, it is the most successful movie in the Disney animated canon for a reason and this was a big part of it.
Cut Song: Plenty. A song for Mufasa to sing called "To Be King", a song for Sarabi titled "The Lion In The Moon", a reprise of "Be Prepared" and "Warthog Rhapsody", which was the original Hakuna Matata.
Warthog Rhapsody made it into the musical, as did the best known cut song, "Morning Report", a song which was also restored to the later DVD releases. "The Madness of King Scar" in the musical is based off of a cut song/scene in the film.
A "work in progress" trailer that was attached to the beginning of one of Disney's The Fox and the Hound video had a snippet of a different version of "Can You Feel The Love Tonight".
Refitted For Sequel: The Lion King originally had a scene where Timon refused to go help Simba and Pumbaa trying to encourage him. The midquel features this instead, in a scene that relates to Timon's overall "dream home" subplot.
Throw It In: Nathan Lane supposedly ad-libbed some of his dialogue, including the "hula" line.
Troubled Production: A director fell out due to conflicts with the creative team. The script needed much fine-tuning. And an earthquake hit California during the final months of production, causing the animators to work at home.
What Could Have Been: Shenzi was originally supposed to be voiced by Tommy Chong, and would have reunited Chong with his former comedy partner, Cheech Marin. Chong turned down the part, and the character was changed into a female, with Whoopi Goldberg performing the voice. Cheech and Chong finally did reunite, however, in 2008.
In 1990, Nala had a younger brother named Mheetu◊, there was fox character named Bhati◊, there was supposed to be a Meerkat character early in the film◊ named Tesma, and Simba's early cub designs looked different◊; Scar was unrelated to Mufasa at first, but they decided it was more emotional if he was family instead of a rogue. Pumbaa was originally named "Harold" and Nala was "Nadra". Nala's father was supposed to be a character. For the sequel, Kovu was meant to be Scar's son until they realized the implications.
Kiara was originally named Aisha, but Power Rangers beat Disney to the punch.
She was also later named "Shani" before we got her final name. Shani had a brother named "Chaka", who was scrapped from the final story.
Mufasa's stint as Rafiki's Spirit Advisor was originally planned to be much more obvious, with the former Lion King's ghost actively speaking to the shaman. Wisely realizing this might come across as unintentionally hilarious, or at least a breaking of the Willing Suspension of Disbelief, the directors decided to instead have all interactions between Rafiki and Mufasa symbolized by the blowing of the wind. This fact is later emphasized when the wind blows around Simba, just before he listens to Kiara and accepts the Outlanders back, and explains why he looks so happy and on the verge of tears at the end, even before he hears Mufasa say "Well done, my son."
The film was originally Darker and Edgier. It was to be themed around a baboon-lion war. During this period Sarabi had a sister named Naanda (who was Nala's mother at the time), Scar (a rogue at the time) was the leaders of the baboons, and Rafiki was a cheetah. There was a hyena named Banagi who was the leader of the Hyenas and there was a pride called the "Mbala" pride; the priderlanders were the "Ndona" pride. Kwashi was a baboon similar to Rafiki, though not as wacky. Noticeably it was much more violent and incest filled than the final product.