In part 7 of From Star Wars to Jedi: The Making of a Saga, George Lucas discusses the characters of a galaxy far, far away.
According to Lucas, actors play a huge role in creating characters for film. "Threepio is just a hunk of plastic," he says, "and without Tony Daniels in there, it just isn't anything at all." Originally, C-3PO was meant to be more of a used-car salesman; Daniels' fussy-servant interpretation was so good, it forced Lucas to abandon his original idea.
Since Lucas killed off Obi-Wan Kenobi in the first Star Wars film, he had to create a new character for Star Wars: Episode V The Empire Strikes Back who could train Luke Skywalker -- and someone who would be interesting to watch. Enter Yoda: small, alien, and green, with a backwards way of speaking. Lucas was apprehensive, worried Yoda -- a puppet voiced and manipulated by Frank Oz -- could be a disaster. Thankfully, it turned out much better than that. "When it goes on the screen, it's magic," he says.
In Star Wars: Episode VI Return of the Jedi, Lucas introduced yet another set of intriguing characters: Ewoks, a deceptively cute species who had the power to overthrow the Empire.