Dawson Casting: It's averted in the film, but played straight in the TV show. Even though Wasabi is younger than Aunt Cass, his TV actor Khary Payton is two months older than her actor Maya Rudolph. His film actor, Damon Wayans Jr., is about 10 years younger than Rudolph.
Defictionalization: Canadian robot engineer Erin Kennedy created a real-life version of Hiro's neurocranial transmitter headband, which was exhibited at Robo Games 2015.
Exiled from Continuity: When adapting Big Hero 6 from an obscure Marvel comic to a Disney animated movie, both Disney and Marvel mutually agreed to divorce Big Hero 6 from the Marvel Universe entirely. Not only does the movie not reference any elements of Marvel canon outside of a Stan Lee cameo during the credits, but Marvel exiled the team from the comics as well, to the point where Marvel has stated they'll never reprint the old comics, much to the dismay of fans of the source material who also like the film. The team's last appearance in Marvel continuity was in 2012, when they appeared in the Spider-Man storyline Ends of the Earth. Whether the characters of Sunfire and Silver Samurai are exempt from this is yet to be seen, as they were excluded from the movie due to being X-Men characters, which 20th Century Fox (which wouldn't be bought by Disney for another few years) had exclusive film rights to at the time of development.
God Never Said That: The voice actress for GoGo Tomago, Jamie Chung, stated that she thinks that Go Go's name is "something plain, like Ethel, Marge or Patty". This quickly warped into "GoGo's VA said that her real name is Ethel".
Missing Trailer Scene: Many of the clips from trailers where Hiro is working on Baymax' red suit are gone from the actual film, notably the famous one where he makes Baymax strike an awesome pose only for the armor to pop right off. Neither do we see Baymax chasing the soccer ball.
Genesis Rodriguez (who is of Cuban and Venezuelan descent) voices Honey Lemon in both the original English recording and the Latin American Spanish dub.
Patrick Stump is the singer of “Immortals” in both the original English recording and in the Italian dub.
T. J. Miller improvised most of Fred's lines, which makes the "Fred's Angels" song even funnier in retrospect. This means a lot of the other characters' reactions were also improvised, like Wasabi saying, "Fred, I will laser-hand you in the face!"
The script simply described Baymax's fist bumping as him doing a robot sound equivalent of an explosion, and Scott Adsit came up with "Bata-lata-lata-la." The creators initially believed It Will Never Catch On, but then they hilarious and tried to insert Baymax fist bumps at every opportunity as a Running Gag. In fact, Hiro fist-bumping the remains of Baymax's rocket fist was not in the initial cut of the film; story lead John Ripa suggested it upon watching an early screening.
A lot of Maya Rudolph's facial expressions while reading lines were copied for Cass, especially the faces she makes when talking about her extra-spicy hot wings.
Cass Hamada initially had a tattoo on her forearm, but it was removed from the final film. Gogo would also have had tattoos.
Hiro's cat, Mochi, was also supposed to get an "upgrade" and join the team as Rocket Cat, but this was scrapped and instead used as a quick gag in the "nerd lab" scene.
Early concept art showed that Yokai had his own team of supervillains (including Mr. Yama and his goons). He also would eventually be consumed by microbots, which would take on his shape and act as the real final villain.
Originally, GoGo was a very cynical character: she disliked Honey Lemon because she thought her fashion sensibilities were just for boys and attention-seeking, and did not approve of Hiro ordering her what to do when they formed Big Hero 6 because she always wanted to be number 1.
The 2013 Rotoscopers video and early information regarding the plot depict Hiro with a slightly different backstory: he was already attending classes with Tadashi, and could not fit in with the other students due to his intelligence (though he doesn't seem to mind). The Rotoscopers video also depicted Hiro as part of a "science club research team" led by Tadashi (most likely consisting of Honey, GoGo, Wasabi, and Fred) in which each individual would make different things and would eventually lead up to a competition. Like Tony Stark, Hiro felt pressure over whether to use his intelligence for good or just build battle robots in the basement (garage in the final film). Also, in early stages of the story, Hiro was responsible for the creation of Baymax, as opposed to Tadashi, just like in the comic book. And the early plans for Wasabi were to make him the quiet, strict character with talent for being a sushi chef like in the comics. Cass was also originally planned to be Hiro and Tadashi's mother rather than their aunt.
The villains Mr. Sparkles, the Bonsai Bombers, and the Fujitas were set to be in the film, but were scrapped. A redesigned Mr. Sparkles appeared in the TV series continuation (his original design can be seen in the film on a poster in Hiro's room◊), and the hostess at the robot fighting match in the opening does resemble one of the sisters.
Originally, Fred was going to use Hard Light holograms which he shaped into Kaiju, an homage to his ability in the comics where he could project an aura shaped like a giant monster.
Man of Action Studios actually wanted to revisit the characters with a cartoon, similarly to what they had recently did with M. Rex (which became Generator Rex). However, they soon learned that Disney was already in development with a feature film. Man of Action had no creative involvement with either the film or the animated series that followed.
Baymax originally looked a LOT different◊. In fact, concept art◊ shows he was originally going to look identical to Tetsujin 28 (on the upper-left).
Word of God: Scott Watanabe confirmed that San Fransokyo is indeed San Francisco during an alternate future that split off after the 1906 San Francisco Earthquake (retconned to the Great Catastrophe in The Series), rebuilt by Japanese immigrants using techniques that allowed for movement and flexibility in a seismic event.
