Producer and co-director Marc Adler says thousands of guesses were cast; entries included Tom Cruise and Brad Pitt.
Delgo is a rarity: An independently produced computer-animated movie made outside Hollywood. Fathom is based in Atlanta.
Still, the risky venture has lured stars including Jennifer Love Hewitt, Chris Kattan, Anne Bancroft, Michael Clarke Duncan, Val Kilmer, Kelly Ripa, Eric Idle,Burt Reynolds and Louis Gossett Jr.
Prinze, who also did voice work for the 2005 animated movie Happily N'Ever After, says: "You expect (Delgo) to be very sort of vanilla, and he isn't. He's a fairly complicated guy."
Delgo is set in a magical world where two warring factions must be saved by the young and impulsive hero and his band of unlikely friends.
Prinze says he was attracted to the project because "I've always sort of been in love with that sort of world, fantasy, ever since I was a little boy with Dungeons & Dragons."
Prinze will take a hiatus from "younger" roles this June when he begins filming Dolan's Cadillac, based on a Stephen King short story, with Gabriel Byrne.