In the original trilogy there are a large number of aliens throughout the film. Now days, the process of creating these creatures is shared by the writers, director as well as art, costume and other departments. In ‘making of documentaries’ you see people sitting down looking over different designs and tweaking them.
What was the process for the original 3 movies, given a sci-fi film of this type & scale with so many strange creatures had never been done before? Did George Lucas sketch out or describe all the aliens? Or was it down to other people to give us the likes of Jawas, Admiral Ackbar, the Twi'lek and the myriad of other alien species throughout the galaxy?
In order to try and make this less open ended, I am looking for an answer on the process rather than specifics about who designed individual races. Those may be future questions depending on this answer. I am also happy to split this into 3 questions if the answers are different for each of the original trilogies.