Unlike the original book, the Once-ler is shown fully in the story as a human. Executive producer Christopher Meledandri said of the change, "The minute you make the Once-ler a monster, you allow the audience to interpret that the problem is caused by somebody who is different from me, and it ceases to be a story that is about all of us. Then it's a story about, 'Oh I see, the person who led us into the predicament is not a person. It's somebody very, very different.' And so it takes you off the hook."
The original name of the town in the movie is Greenville (as can be seen in the scene where the Once-ler is selling his thneeds in the town center, "Greenville Circle") before the commercial success of the product changed the name to Thneedville.
The film boasts more than seventy product integration deals, a rather high number considering the ecological message that the film imparts. One of the partnerships, with Mazda, was particularly singled out for being ill-judged.
When the Once-ler is walking to his office once he became rich, what appears to be a blow-up magazine cover is hanging on the wall. It is caption "Too big to fail", which is a colloquial term in economics to define an institution whose failure would bring catastrophic consequences to the economy. After the 2009 banking collapse, modern economists have said that institutions too big to fail are simply too big to exist.
Although they go uncredited because they have no lines of dialogue at all in the movie, O'Hare's henchmen are called Buddy and McGirt. O'Hare calls them by name only once, when he orders them to search Ted's room for the Truffula seed. Together, they are named after retired champion boxer James Walter "Buddy" McGirt, who during his boxing career was the IBF light-welterweight champion, and Lineal and WBC welterweight champion.
In Despicable Me (2010), Margo wore a Lorax shirt as a possible teaser to this film. A similar thing occurred in Despicable Me 3 (2017) where she wore a Grinch shirt as a teaser to another Dr. Seuss film from Illumation, The Grinch (2018).
The trivia items below may give away important plot points.
The soundtrack reflects the natural vs. man-made nature of the scene. For example, the opening song "Thneedville" describes the thoroughly artificial town with completely electronic music. Meanwhile, the finale "Let It Grow" and the planting of an all-natural tree is made with almost all acoustic instruments. "Let It Grow" even includes a capella choral music (with no man-made instruments at all).