Bob Peterson wrote Dug's line "I have just met you and I love you" after being inspired by the words of a small child he met while working as a camp counselor in the 1980s.
Young Ellie was voiced by Elie Docter, the daughter of director Pete Docter.
Most of the sounds Kevin makes belong to director Pete Docter.
Carl notices that Russell has never pitched a tent despite all his badges; the fact that Russell does not ask his father for help implies that his parents are divorced. This is the first instance in a Disney animated film of divorce being shown as the cause of single parenthood.
It is the first film for Jordan Nagai, who voiced Russell. His older brother Hunter was attending the auditions and Nagai went along with him; out of 400 children he stood out for his talkativeness, and the producers found him ideal for Russell.
Carl Fredricksen's face and grumpy personality were inspired by Spencer Tracy and Walter Matthau.
In the childhood scenes only Ellie speaks; in their adulthood only Carl speaks.
According to Pete Docter, Carl and Charles Muntz are mirror images of each other, down to their names. Both have suffered a great loss and become obsessed with the past; the difference is that Muntz's obsession makes him ruthless, while Carl learns to let go of the past thanks to Russell and Dug.
There is less than a month's age difference between Edward Asner (Carl) and Christopher Plummer (Charles): Asner was born on November 15, 1929, and Plummer on December 13, 1929. Both died in 2021, at the age of 91, seven months apart.
Bob Peterson created Dug's voice by drawing on the way he talks to his own dogs.
Russell is Pixar's first Japanese/Asian-American character voiced by an Asian-American actor.
Carl never speaks directly to Ellie throughout the film; the only exception is when he talks to her while pointing out the baby-shaped cloud before a car tire bursts.
Russell's appearance was inspired by a caricature of Korean-American Pixar employee Peter Sohn.
Carl Fredricksen is 78 years old; Ed Asner was also 78 when he recorded his lines.
All the dogs except Dug are named after letters of the Greek alphabet. When Dug puts Alpha in the Cone of Shame, the other dogs begin calling him Alpha. Dug and Alpha are voiced by the same actor. Alpha, Beta and Gamma are a reference to the working classes in Aldous Huxley's novel Brave New World.
When Beta and Gamma laugh at Alpha's squeaky voice, Alpha implies that they are challenging his rank. Beta denies it but says Dug could do it. At the end of the film Dug puts Alpha in the Cone of Shame and becomes the new Alpha.
Nurse AJ is the first Pixar character with a bangs hairstyle.
Dug is Bob Peterson's favorite Pixar role.
Carl's square face and body emphasize how boxed in he has become in his life and his home.
Christopher Plummer is the eighth Academy Award winner to voice a Disney villain. The previous ones: George Sanders (Shere Khan), Peter Ustinov (Prince John), Geraldine Page (Madame Medusa), Jack Albertson (Amos Slade), George C. Scott (Percival C. McLeach), Jeremy Irons (Scar) and Kevin Spacey (Hopper).
Gamma was voiced by Jerome Ranft, the brother of Joe Ranft, who died in 2005; Joe was also said to have been intended for the role.
Up, Christopher Plummer's third animated film, was also his first computer-animated, non-Don Bluth and PG-rated film. His previous two films were An American Tail and Rock-A-Doodle.
All of Charles Muntz's dogs except Dug had their tails docked. Those with docked tails were obedient, while only Dug showed love and kindness.
Carl becomes grumpy after his wife's death, but regains his old cheerfulness at the end of the film. Christopher Plummer voiced Muntz; he had previously played Captain Von Trapp, who grows cold after the death of his first wife, in The Sound of Music.
In the French and French-Canadian versions, Carl Fredricksen was voiced by Charles Aznavour.
Four months after Up, Christopher Plummer's first CGI film in 2009, he appeared in the film 9.
As of 2018, Up was the only film Pete Docter worked on or directed in which his voice work was not credited merely as "Additional Voices."
Charles Muntz's dog Beta was inspired by Snots (a Rottweiler), Cousin Eddie's dog in National Lampoon's Christmas Vacation.
Up is one of the two PG-rated Pixar films featuring John Cygan; the other was Inside Out. Cygan was said to be set to appear in Incredibles 2, but he died of cancer in 2017.
It is Pixar's fourth film in which the director or co-director has a major role: Pete Docter voiced Kevin, and Bob Peterson voiced Dug and Alpha. The other three films: Finding Nemo, The Incredibles and Cars.
In the Brazilian dub, Carl Fredricksen was voiced by Chico Anysio, Charles Muntz by Jomeri Pozzoli, Russell by Eduardo Drummond and Dug by Nizo Neto.
In the Spanish dub, Carl Fredricksen was voiced by Luis Varela, Charles Muntz by Claudio Rodríguez, Russell by Julian Mora and Dug by Fernando Cabrera.
Up was the sixth animated film in which studio employees voiced the main characters and were listed in the cast outside the credits. The previous ones: Lilo & Stitch (2002), Open Season (2006), Meet the Robinsons (2007), Ratatouille (2007) and Bolt (2008). It was also the last film of this kind in the 2000s and, until Early Man in 2018, the only production of this kind that was not a sequel or a spin-off.
Russell's name was initially conceived as Lewis, but it was changed to avoid confusion with the main character in Disney's Meet the Robinsons.
Bets, one of Muntz's dogs, and the dog voiced by John Cygan were the only characters voiced by professional actors rather than Pixar studio employees.