Carpentry was basically a way for him to avoid having to take roles he didn’t want, like many of those guest-star appearances.

[...]

So Lucas agreed to bring in Ford, but only for the sake of giving the Luke and the Leia of that group someone to read lines with (I don’t know who his initial group was, as I believe Lucas mixed and matched as he went along, wanting to see how the actors would work in different configurations).

[...]

In the aforementioned Reddit AMA, Ford recalled the situation: “I had helped George Lucas audition other actors for the principle [sic] parts, and with no expectation or indication that I might be considered for the part of Han, I was quite surprised when I was offered the part. My principle [sic] job at the time was carpentry.”

[...]

Lucas was adamant about not re-using actors from “American Graffiti.” ([...]. Lucas didn’t want to have a stable of actors like, say, Martin Scorsese has a group of actors he works with a lot, like Robert De Niro.) Soon, though, at the urging of everyone around him (including his good friend, fellow director Steven Spielberg and, of course, Roos), Lucas began to seriously consider Ford for the role, as his line readings were continually better than the actors who were “really” auditioning for the role.