Neither #1 nor #2. Coming out of acting school, Mark came under the wing of Arthur Hill ("Owen Marshall MD"), who used his connections to get Mark TV roles. When the parts for Star Wars were being filled, Lucas thought the way Kubrick did for 2001, that he wanted mostly actors with experience but not big names, ending up with Ford--who like Hamill had TV experience in various shows--and Fisher. Getting Gielgud and Cushing were strokes of genius, creating a balance between the old and the young in acting, making the younger trio look more like a galactic version of the three musketeers.
He’s not the first weirdo to land a big part..
Casting directors are famously hit or miss when it comes to young actors.
Sometimes they stay cute when they grow up but often you get a Mickey Rooney that grows up to be a freak.