Castile was born in Orange County, California. One of his earliest television appearances was in a long-circulated segment of Totally For Kids, kid safety interstitials that aired on the Fox network's Saturday morning lineup in the early 1990s. In one segment, he portrayed a young boy who was followed by a group of kids (the main characters of the "safety patrol") because he had his name printed on his shirt, as a way to address the dangers of name display for strangers (or possible abductors) to see. The Totally For Kids segments also featured John Walsh.

Shortly after, Castile appeared in the Miller-Boyett sitcom Going Places in early 1991, playing the recurring role of Sam Roberts, the son of talk show host Dick Roberts (Steve Vinovich) with co-stars Alan Ruck and Heather Locklear. Castile and his fellow Going Places cast member Staci Keanan were soon moved by Miller-Boyett into their forthcoming sitcom Step By Step, that premiered on ABC in the fall of 1991. He and Keanan played brother and sister Mark and Dana, on the Foster family side, for the entire run of Step by Step, which moved to CBS in 1997 and then ended in 1998.

Meanwhile, in 1992 and 1993, he played Ted Newton in the films Beethoven and Beethoven's 2nd. In 1996, while continuing on Step by Step, Castile added the vocal role of Eugene Horowitz on the animated series Hey Arnold!. Castile left Hey Arnold! after eight episodes; he was replaced as the voice of Eugene by Jarrett Lennon, the actor originally cast as Mark Foster on Step By Step in 1991 before the producers replaced him with Castile.