It was AnnaSophia Robb who came up with the name of the Dark Master. Initially in production, it was referred to as the "Key Monster."
Because the 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 film's later scenes.
Due to the fact that there are no squirrels in New Zealand, all squirrel shots were either prerecorded or digitally added in.
When AnnaSophia Robb heard of the film being made, she wrote a letter to the director Gabor Csupo expressing her love and enthusiasm for one of her favorite books. This letter effectively won Robb the part of Leslie.
The name "Terabithia" is inspired by an island called Terabinthia in C.S. Lewis' Chronicles of Narnia novels.