According to an interview at MTV, Pixar‘s Up was originally an even more overt adventure movie, as a subplot featured Carl Fredricksen protecting a giant egg (laid by the bird Kevin) that is believed to be a sort of fountain of youth. The fact that a subplot was dropped from the film isn’t a big deal, as it happens all the time in many films of all types during development, but this detail provides an interesting glimpse into the film as it was originally conceived.
“That was early on,” director Pete Docter said of the subplot that had Carl constantly cradling a giant egg. “We had the bird [Kevin] give birth to this egg, and Carl then felt like he needed to take this baby — it needed tending to, and needed to get home.” That gives Carl additional motivation to evade the dogs which hound him through a section of the film. But the deeper point, and the one which would have altered the feel of the film more noticeably, is that the antagonist Charles Muntz was desperate to get his hands on the egg, which “had a larger picture involved with Muntz, [who] wanted the egg because it was this youth potion that made you grow young.” Carl didn’t know about that aspect of the egg — he didn’t want the thing’s youthful powers for himself — but knew he had to prevent Muntz from getting it. So what happened? “[Muntz] was after the bird and all that; it was really interesting,” Docter explained. “But it kind of got more bizarre… it was an element that we dropped out from the story.”
From a screenwriting perspective, adding a MacGuffin like the egg seems like an obvious move, especially within the context of a story that already has a huge adventure component. You get the tension of Carl not knowing what he’s got, wondering what he’ll do when/if he learns the truth. But the film is stronger without it. Having Muntz want to literally become young again is a bit too directly drawn from traditional pulp adventure. This way, he and Carl are both attempting to fulfill promises they made in their youth, but we get to concentrate on the divergent ways that their lives have affected their methods, rather than on an empty object of attention.