I have absolutely no facts with which to back up the following assertion, but I’ve often wondered if young George interacted with his young actors differently in 1976 than old George did in 2001. In ‘76, he was “George Lucas,” with one hit film and one artsy indie sci-fi film under his belt, and not that much older than Hamill and Fisher (and actually younger than Ford).
Contrast that with 2001. He’s now “GEORGE LUCAS,” a true mogul and an instantly-recognizable part of Western culture. He’s been around the block, maybe feeling himself a bit more nowadays, and is much older than his two principal leads.
Did he feel less inclined to collaborate with the “kids” in ‘01 than he may have in ‘76? Was he more willing to accept and play with feedback from his young actors in ANH than he was in AOTC, where he might have been more inclined to just tell Portman and Christiansen “this is what I need you to do”? And were Portman/Christiansen perhaps so awed and even intimidated by working with GL whereby they just “did as they were told,” vs. pushing back on the crap dialogue?
Also, I’m going to throw in Ian McDiarmid there as another PT actor who could spin straw into gold. Once he became the Emperor, there was some pretty goofy scenery-chewing, but up until then, some really great work.