Katherine Paterson wrote her novel in 1977 as a way of helping her 8-year-old son recover from a traumatic incident - his best friend was struck by lightning and killed. The son - David Paterson - grew up to become the film's producer.
In the scene where Leslie's telling Jess about Janice Avery's home life while walking to a swing under the tree house, Jess has a slight limp. This is because during filming, Josh Hutcherson developed stress fractures in his back from constantly playing sports and being overactive.
Because shooting was not done in sequence, a "make believe" scale of 1 to 10 was used so Josh Hutcherson could gauge how much computer generated imagery Jess would be seeing. They rated Jess' view of Terabithia from the treetops as level 10. Similarly, the five stages of grief emphasized in the book (denial, anger, bargaining, depression, and acceptance) were used so Josh could appropriately focus his performances in the later scenes of the film.
The scene where Jess and Leslie are racing a group of Hairy Vultures to the tree house was shot with "just the two of them on a treadmill against a green screen roped up like rock climbers" according to screenwriter Jeff Stockwell.
In the scene where Jess and his father are playing with the set of cars, the song on the radio is "You've Got To Keep Your Mind Wide Open", which is the song featured in this movie's music video, performed by AnnaSophia Robb.
Josh Hutcherson considers himself to have a horrible singing voice. Regardless, due to the heavy accents of most of the New Zealand extras, the singing was added as ADR (Additional Dialog Recording) in post-production and is listed as "performed by Zooey Deschanel and the Terabithia choir" in the end credits.
The emotional scene at Terabithia where Jess is picked up and cradled in his father's arms after imagining he is being chased by the Dark Master and falling, was fourteen takes on a full day's shoot broken in the middle by lunch.
When she heard of a film being made, AnnaSophia Robb wrote a letter to director Gabor Csupo expressing her love and enthusiasm for one of her favorite books. This letter effectively won Robb the part of Leslie.
The negatives of the wide shots of Ms Edwards driving Jess to the museum were flipped so that New Zealand traffic driving on the left side of the roads would appear to be driving on the right side of the roads.
The film attracted some criticism for the way it was marketed - as a rich fantasy adventure set in a mystic land. The reality is that the majority of the film is set in the real world, showing how children cope with the harsh truths of the life. The fantasy elements comprise just a small fraction of the movie's running time.
The Dark Master evolved from the Hornapoligist, a rhinoceros-like creature, according to screenwriter Jeff Stockwell. In the film, the Hornapoligist only appears as a drawing (in the background of the end credits).
The pest trapped in the greenhouse was meant to be an opossum, but they are not indigenous to New Zealand and the production was not permitted to import them for the shoot, so a Common Brushtail Possum was used instead, which is native to Australia and was introduced in New Zealand by early settlers as a source of meat and furs.
The trivia items below may give away important plot points.
In initial meetings, David Paterson discovered that studios were loathe to touch the project because Leslie gets killed. It was suggested to him that she should have a mild accident or be briefly in a coma instead.
It is hypothesized that Jess's dad cut the rope that Leslie fell from. After being upset with Jess having his "head in the clouds" he cut the rope to stop the kids from playing together again. It is also theorized that Jess doesn't have to do his chores afterwards because of the grief his father felt about cutting the rope.