The Very Beginning
Mark Hamill said that he found the prospect of working with Alec Guinness very intimidating. The latter was an established legend while Hamill was at the beginning of his career. However, he said that the veteran British star was always a gentleman and had great patience for his colleagues.
It must have helped that Hamill is one of the most likable people on the planet. He always has a smile on his face and wanted to create a positive atmosphere on the set. The young star had a good relationship with Guinness and this shone through the movie (via CNet).