Casella has growth hormone deficiency, which accounted for his ability to play characters much younger than his actual age.

[10] In 1992, Casella played Racetrack Higgins in Newsies, which was based upon true events in the 1899 newsboys strike against Joseph Pulitzer and William Randolph Hearst.

He appeared in WindRunner: A Spirited Journey, starring Margot Kidder and Russell Means as Jim Thorpe's ghost.

He also voiced Tip in The Little Mermaid II: Return to the Sea, Zini in the CGI film Dinosaur and the titular character in ABC's Cro.

In 2008, Casella played Mack Steiner in Leatherheads and Dennis, one of the main characters in Scaring the Fish.