If you read the novel first, as I did, I think you look at it from a different perspective. Fontane was certainly a major character, almost like another son to Vito, and, as such, held my interest. He was also a vehicle used to illustrate the power of the Don, as when Vito gets him released from his contract with the bandleader, "arranges" his movie role with Woltz, and finally fixes his Academy Award win. Finally, I think Puzo may have had something (metaphors? contrasts?) in mind in using the fim industry and organized crime in counterpoint.

If you see the film first, then Fontane is a minor character, and it's understandable why the lengthy parts about him in the novel are boring.