Casper is expressed in the second season by Robbie Sublett in the film , Matthew Géczy in the first season and Devon Werkheiser . Casper is voiced by Robbie Sublett in the film , Matthew Géczy in the first season and Devon Werkheiser in the second season . Casper is voiced by Devon Werkheiser in the film , Robbie Sublett in the first season and Matthew Géczy in the second season .