Peter Mayhew was eligible for the roles of either Chewbacca or Darth Vader. He chose Chewbacca because he wanted to play a hero.

Star Wars (film)

See also: List of Star Wars characters and List of Star Wars cast members

From left to right: Luke Skywalker (Mark Hamill), Princess Leia (Carrie Fisher), and Han Solo (Harrison Ford)

Mark Hamill as Luke Skywalker: a young man raised by his aunt and uncle on Tatooine, who dreams of something more than his current life and learns the way of a Jedi.

Lucas favored casting young actors who lacked long experience. To play Luke (then known as Luke Starkiller), Lucas sought actors who could project intelligence and integrity. While reading for the character, Hamill found the dialogue to be extremely odd because of its universe-embedded concepts. He chose to simply read it sincerely, and he was selected instead of William Katt, who… Continue Reading


Source: https://en.wikipedia.org/wiki/Star_Wars_(film)#Cast