I think one of the best choices Lucas ever made for Star Wars was to cast by ensemble chemistry and then stick with them
I can picture pre-production on Empire like "Hamil screwed up his face and isn't pretty anymore," and Lucas be all like "No, Luke got punched in the face by a giant monster."
Or Jedi where it must have been "Carrie is coked up half the time, we have to replace her," and I can imagine Lucas as "No, that's Leia who watched her entire home planet blown apart after betraying everything she believed in. Anyone might need a little help getting through the day, after that."
And finally with Force - "I don't care of Mayhew decided having his picture taken is against his religion, and he is living in a yurt on top of a mountain. Someone go get him, because Chewbacca is due on set Tuesday at 6!"