James Belushi went to producer Edward S. Feldman's office and trashed his desk in protest of this film. Feldman was not in his office at the time but Belushi told Feldman's secretary "Tell him I was here".
Michael Chiklis was chosen over 200 other candidates for the role of John Belushi. Chiklis first auditioned in 1985 and was finally cast in 1988 after numerous rounds of auditions. This was the first film Chiklis auditioned for.
Because the film is an "unauthorized" biopic, many of the skits and song numbers performed on Saturday Night Live were copyrighted by NBC, so in the film, some were made-up and not in fact actual skits and numbers John performed (i.e. "The Samurai Baseball" skit and the "634-5789" Blues Brothers number). Similarly, where Belushi famously mocked Joe Cocker's performance of "With a Little Help from My Friends" at Woodstock, in the film Cocker's later hit "You Are So Beautiful" is performed.