 He played an alienated young man, the son of a prosperous automobile dealer, who drifts into petty thievery.His breakthrough role was in the title role in Francis Ford Coppola's 1966 comedy "You're a Big Boy Now", which starred Elizabeth Hartman, Geraldine Page, Rip Torn, Karen Black, and Julie Harris, in which Kastner played an earnest young man who moves from his parents' house to New York City and struggles with confusing relationships.He played a similar role as an earnest young advertising man swept up in the era in 1971's "B.S.