“Harrison Ford was a carpenter who became an actor” IS NOT TRUE.
He was an actor who did carpentry on the side, like some actors tend bar, to earn money when he couldn’t get acting jobs. He wasn’t a carpenter who they spotted and turned into an actor. He wouldn’t have read the other actors during the casting of Star Wars if he couldn’t act. He’d already been in American Graffiti and The Conversation, for Lucas and Francis Ford Coppola, as well as lots of TV guest shots, before Star Wars.
Also Mark Hamill had done dozens of TV guest shots, a soap, and was in a prime time series (and would have been in Eight is Enough except for the car wreck) before Star Wars. It wasn’t his first role either. It was his first live action movie.
Carrie Fisher had done one movie before Star Wars (Shampoo) but she’d been in her mother’s stage act and was in drama school.
(this has been a public service announcement)