When a younger Charles Muntz speaks to a large audience that he will return with the beast alive, everyone is wearing a hat. What the viewer can't see, however, is that he is speaking to a literal "Sea of Hats". There are no people under those hats (DVD director's commentary).
The villain Charles Muntz is named after Charles Mintz, the Universal Pictures executive who in 1928 stole Walt Disney's production rights to his highly-successful "Oswald the Lucky Rabbit" cartoon series. This led Walt Disney to create Mickey Mouse, who soon eclipsed Oswald in popularity.
When Russell goes to rescue Kevin, he catches up with the airship, the Spirit of Adventure. The shot of Russell - from the inside cabin, is of some of the dogs playing cards (using Milk Bones as their chips), is a nod to the iconic, (famous painting; Dogs Playing Poker.
About halfway through the film, Carl and Russell are trying to put up a tent. Carl is surprised that Russell, with all his badges, never put up a tent before. When he asks Russell why he didn't ask his dad for help, Russell reveals (albeit indirectly) that his parents are divorced (during the conversation where Russell says "Phyllis isn't my mom"). This is the first time in a Disney full-length animated feature where divorce is even hinted at as being a reason for a character having a single parent (most of Disney's single parents are assumed to be widowed).
Doug's 'point' pose, where his entire tail, back, and head is in a perfectly straight line, is an homage to the identical pose that Mickey's dog Pluto often makes. Doug also shares a similar color scheme to Pluto.
All characters are based upon circles and rectangles, except for the villains who are triangles. Not only are Carl and Ellie based on squares and circles, but objects around them are based on their shapes, like their chairs and picture frames. When they both appear in a photograph, the frame is both circle and square.
Pixar is known (at least by devoted Pixar fans) for referring to a character in their next movie to come out in their most recent one. A stuffed Lotso bear (from Toy Story 3 (2010)) appears (along with the ball from Luxo Jr. (1986) and the plane from Toy Story (1995)) in the room of a little girl Carl passes when his house first takes off.
The very first animated film, as well as the first 3-D film, ever to open the Cannes Film Festival. When the film was over, the Festival audience remained completely silent. During a panel at the 2011 D23 Expo, executive producer John Lasseter said that it was actress Tilda Swinton who broke the silence by applauding and leading the audience in a standing ovation.
Film debut of Jordan Nagai, who voices Russell. Originally, his older brother Hunter was auditioning for the part, and Nagai simply came along with him. About 400 children had showed up for the auditions, but Nagai stood out because he would not stop talking. Director Pete Docter later said that "as soon as Jordan's voice came on we started smiling because he is appealing and innocent and cute and different from what I was initially thinking."
When we first meet Carl as a child, the left side of his collar is sticking out of his vest while the right side of his collar is tucked into his vest. When we first meet Russell, the left side of his collar is tucked under his neckerchief and the right side is sticking out.
The house is based on the real life Edith Macefield house in the Seattle suburb of Ballard, Washington. Edith fought building developers and her little house still stands in the center of a large development known as the Ballard Blocks.
The term 'A113' is the number of the courtroom, and can be found on the gold sign Carl sits next to while waiting to be called (Courtroom A113). A113 is a frequent Pixar in-joke based on one of the room numbers for the animation program at Cal Arts.
If Carl's house was approximately 1600 square feet, and the average house weighs between 60-100 pounds per square foot, it weighs 120,000 pounds. If the average helium balloon can carry .009 pounds (or 4.63 grams), it would take 12,658,392 balloons to lift his house off the ground. (20,622 balloons appear on the house when it first lifts off.)
Russell's Wilderness Explorer sash has several in-jokes and tributes. The most obvious is a Luxo Jr. (1986) ball, which can also be seen on the floor of the room of a girl watching Carl's house float by. One badge has a hamburger with a candle in it. This is a nod to Merritt Bakery in Oakland - which creates cakes in that shape - a favorite hangout of director Pete Docter and producer Jonas Rivera. Another badge is a tribute to 2-D animation, showing a perforated paper that is used by 2D animators to line up their drawings correctly. He also has badges for First Aid and Second Aid, which may be a reference to a short on the Up website where Russell struggles to apply bandages to Carl. Yet another badge depicts a multicolored pinwheel - the "hang" icon of Apple's Mac OS X operating system Windows uses an hourglass icon). Several of the badges are shown in the credits. An additional tribute to Apple and Steve Jobs (former CEO of Pixar) shows Russell trying to teach Carl how to use a computer. The font used for the numbers on Carl's alarm clock is the "Chicago" font, one of the first fonts designed for the Macintosh. Steve Jobs, also spearheaded the original Macintosh project at Apple.
When Carl is watching television, and is interrupted by Russell knocking at the door, he is watching a home shopping channel. This particular program has become a well-known blooper video of a pitchman making a gaff in which he describes a picture of a horse, except the picture he is describing is actually that of a moth.
The rifle that Charles Muntz uses is an 1874 Sharps, a very popular model with buffalo hunters of the American Wild West, and the procedure he uses to load, aim, and fire the weapon is accurate. (His use of shot-shells in a long-range rifle, however, is questionable at best.)
A subplot involving Carl keeping one of Kevin's eggs (Which could reverse the aging process) from Charles Muntz was conceived in the early stages of production, but never scripted, due to it being (in director Pete Docter's words) "too bizarre".
The victory music that plays in UP when Dug outwits Alpha and puts the cone of shame on him sounds like the victory music in Finding Nemo when Nigel outwits the seagulls and makes them get their beaks stuck in the sail of a yacht.
During pre-production of the film, director Pete Docter looked up to Disney veteran animators Frank Thomas, Ollie Johnston, and Joe Grant for inspiration. Docter stated that the film reflects the friendship he shared with these three talented animators before their passing as well as wanting to learn what they went through during their years working for Walt Disney and soon after.
On the DVD cover and in commercials, the dogs appear to be flying Curtiss F9C-2 Sparrowhawk airship-launched fighters. US Navy airships such as the Macon and the Akron based at Moffet Naval Airstation between the first and second World Wars were actually designed to carry and launch these fighters.
Rex from Toy Story (1995) makes a nod to this film in Toy Story 2 (1999). When the gang goes to Al's apartment to rescue Woody and try to find a way into Al's apartment, Rex mentions finding balloons to float to the top.
The trivia items below may give away important plot points.
When the dogs start attacking Russell with airplanes at the end, this aerial fight literally becomes a 'dogfight'. Also, the dogs refer to each other with "Grey leader", "Grey One", "Grey Two", etc. This is a nod to Star Wars: Episode IV - A New Hope (1977) (where pilots referred to each other with Red Leader, Red One, etc.), and it also jokingly refers to the myth that dogs cannot see colors, only black, white and shades of gray. Also when Russell distracts the dogs and their planes start to scramble, their formation breaks just like Darth Vader's squadron when they were attacked by Han Solo in the Death Star. Lastly, when the dogs start shooting, their guns sound like the blasts from an X-Wing fighter.
Muntz is the fifth animated Disney villain to fall to his death (following the Wicked Queen [Snow White and the Seven Dwarfs (1937)], Ratigan [The Great Mouse Detective (1986)], McLeach [The Rescuers Down Under (1990)], Gaston [Beauty and the Beast (1991)], and Frollo [The Hunchback of Notre Dame (1996)]). He is the first Pixar villain to do so. It's also noteworthy that Lucifer the cat [Cinderella (1950)] fell to his apparent death but was brought back in the direct-to-video sequel, Malificent [Sleeping Beauty (1959)] falls after being mortally wounded with a sword, and Clayton [Tarzan (1999)] falls after fighting Tarzan, but actually dies by being hanged by vines. Many death scenes were proposed for Charles Muntz; in one of them, his obsession with catching Kevin took him inside the dreaded labyrinth against his own recommendation, where he would eventually get lost and die (much like Jack Nicholson's character in The Shining (1980)). As the animators wanted to keep the climax situated in the air, they considered that Muntz be lured into Carl's house by Kevin, and then die as the house fell off the zeppelin with him still in it. However, they did not want to associate the house, which symbolized Elly, with a violent death. Another ending that almost made it was Muntz getting tangled into some balloons and getting lifted away, instead of falling down. But this did not give a proper closure to the character. In the end, the directors decided that this was Carl's story, and Muntz' ending was therefore to be kept simple.
Inside the newly-updated photo album, one of the pictures of the couple is of them in a car, looking over their shoulders (Carl in the driver's seat). The pose they are in, as well as the car is a recreation of a famous ghost photo. In the picture, the man was posing for his brand new car, and in the passenger seat was the ghostly image of his mother.
All of the dogs except for Dug are named after letters of the Greek alphabet (Alpha, Beta, Gamma, etc) although this could relate to rankings in a dog pack, where the lead male is known as the Alpha, then Beta and so on. This is supported by the fact that when Dug puts Alpha in the Cone of Shame, all the other dogs begin referring to Dug as Alpha. The voices of both Dug and Alpha are performed by the same actor, Bob Peterson. The three main dog characters, Alpha, Beta, and Gamma, as well as being named for the Ancient Greek alphabet, also reference three classes of workers in Aldous Huxley's novel 'Brave New World'. It is also worth noting that Muntz' "chef" is a dog named Epsilon, another class of worker from 'Brave New World', and that the third dog piloting the small planes alongside Beta and Gamma is named Omega (whose name is actually never used anywhere in the film itself, being only used in the Credits, voiced by Josh Cooley).
When Russell flies past the airship using his balloons and the leaf-blower, we briefly see several of Charles Muntz's dogs playing poker at a card table. This is a tribute to the famous "Dogs Playing Poker" series of paintings by Cassius Marcellus Coolidge.
Carl and Russell's hometown at the end is Oakland, California. We see Oakland landmarks and the Fox Oakland Theatre (showing Star Wars: Episode IV - A New Hope (1977)) and Fenton's Creamery. The Fenton's Ice Creamery actually and has two locations, the other in Vacaville, CA, on East Monte Vista Avenue. Both locations are close to Pixar's headquarters in Emeryville, CA. There is also an express version of the creamery in Oakland International Airport to eat while you wait to catch a plane or if you just flew in. Fenton's did not have to pay to have it in there; Disney and Pixar put it in the film for free.
In the closing credits, many of the photos have the same theme as the title of the corresponding crew member, i.e. Music By shows Carl playing water glasses and Russell playing a trumpet, Story Supervisor shows Carl telling a story around the campfire, Film Editor shows Carl and Russell in front of a movie theater showing Star Wars, Production Designer shows Dug and Carl designing pictures on the sidewalk, Technical Director shows Carl dangling a computer mouse, Production Manager shows many puppies had been produced, Supervising Animator shows Dug in three stop frames of animation, Photography, Camera, and Lighting show photo booth photos of Carl and Russell, Shading Art Director shows them doing shadow puppets, and so on.
The "Shady Oaks Retirement Village" seen in the leaflet, is likely to be an in-joke reference to the Touchstone Television series "The Golden Girls", another Walt Disney Company title dealing with older people. Dorothy and her mother Sophia always talked about the dubious retirement home called "Shady Pines".
The third Pixar film to show blood, shown briefly when Carl wacks one of the Construction Workers supposedly damaging his Mailbox with his Cane on the head. The first two are: Finding Nemo (2003) and _The Incredibles (2004)_, not counting A Bug's Life (1998) where it's own a Small Sample/Not shown from someone getting hurt.