George Lucas: "...And that's how the Wookiee came into being"
Alan Arnold: "Now let's take C-3PO and R2-D2."
GL: "It began with R2-D2. Walter Murch and I were mixing American Graffitti, and Walter wanted to make some changes ot one of the sound tracks. He needed the dialogue tracks for reel two and so he said "George, get me R2-D2" , meaning reel two, dialogue two, the second of several dialogue tracks. And I said, "That's a great name, Walter" and wrote it down in my notebook. When we finished on Graffitti I began writing Star Wars , but R2-D2 was still only a name. I wanted two robots in the film and wanted one to be sort of a human type, and the other to be more of a computer type. I also wanted there to be humour in the fact that they were very different in temperament, a sort of Abbott and Costello comedy team - one short and fat, the other tall and skinny. I don't know why, but from the start I thought of R2-D2 as being the msaller one, and then had to think up a name compatible with R2-D2. After playing around with a lot of letter and number combinations, I came up with C-3PO."
Page 224-225 'Once Upon a Galaxy: The Making of The Empire Stirkes Back' by Alan Arnold