Director Csupó stated that they had no actors initially in mind for the film. The first actor cast was AnnaSophia Robb as Leslie Burke. Robb wrote Csupó "such a beautiful, heartwarming letter" that expressed her love for the book and the character. Csupó said that he cast her because of "her letter, her enthusiasm, and her love of the material". Robb also conversed with producer Lauren Levine before casting even began, and "their conversation convinced her that, without a doubt, AnnaSophia was meant for this role". Levine said that "it was just so clear in talking to her about all this fantasy that I was basically talking to Leslie, that she had that same kind of spark and magical presence. She might be physically different from Leslie in the book, but the spirit of Leslie and the spirit of AnnaSophia are nearly identical. It was a match made in heaven." With regard to the character, Robb said "[Leslie]'s one of those people who's just always lit up, who has this glow about her, and no one can bring her down. Leslie's such a lively and energetic character, it was really fun for me to become her."
Levine stated that "looking for Jess was a really tough hunt. We needed someone who could go from an introverted boy in an isolated world to someone who completely taps into his imagination and becomes a confident, brave leader in Terabithia. That's a heck of a range for such a young actor." Josh Hutcherson was not their first choice for the role of Jess Aarons, but they settled with him because they "felt the chemistry between AnnaSophia Robb and him". Hutcherson said that the project appealed to him because of "the real life day-to-day drama as well as the arc of the character Jess".
The filmmakers cast Robert Patrick as Jess' hardworking and strict father based on his previous roles in the films Walk the Line, Flags of Our Fathers, and the television series The Unit. Patrick explained that he related to the story because he was "constantly creating imaginary worlds as a kid" himself, and that the film's setting reminded him of where he grew up. He also said that he took on the role because he wanted to star in a film that his children could watch.
Csupó said that they cast Bailee Madison as May Belle Aarons after weeks of searching for an actress to play the part. He went on to say that "she had such a charm, even before the camera, she was just like a little sweetheart. She was very confident, she showed up shook hands with everybody, totally sweet and perky. I said 'WOW!' she was just stealing everybody's heart on the spot."