In earlier scripts he played a more important role and was more involved in the story, appearing in the middle of the film. Screenplay writer Michael Arndt however found that he was distracting too much from the other main actors and Hamil's role was reduced to a mere MacGuffin. (Central plot motor/motivator but does not really appear).
Idk at what point Hamil saw the first drafts but maybe he first thought he'd play a bigger role. Possibly this was also what the people thought that negotiated his salary.