With Star Wars being a massive phenomenon today, it's strange to think there was a time when nobody knew what it was or how big it was going to be. This includes the people who made the movie.
Mark Hamill said in an interview with StarWars.com he didn't even realize the movie was about Luke at first. He thought he was auditioning to play Han Solo's sidekick.
Hamill says he didn't get to read the full script until he got the part. Before that, he read one scene with Harrison Ford.
"First of all, there was a cattle call, where everybody in town that’s the right age goes in and they look at your size, your height, your looks, your this, your that. And you talk back and forth. They interview you, you don’t really hear them talk about the movie," Hamill said.
"...when I tested, I figured Harrison’s a leading man. So I thought, 'I'm playing his sidekick, right?'"
Hamill's first indication that he would actually play the lead role was the film's working title. (Remember, at this point, Luke's last name was still "Starkiller.")
The title page of the script read, "The Adventures of Luke Starkiller, as Taken from the Journal of the Whills, Saga Number One: The Star Wars." The more he read the script, the more he realized he wasn't Ford's sidekick at all. He landed the lead role.
Soon, Luke Starkiller's name was changed to Skywalker, and the title was shortened to "Star Wars." Probably the right call on both counts.