Via Starwars.com:
Welcome to From Concept to Screen, an ongoing article series about the various stages of production that your favorite character, vehicle, creature, location, or scene of the Star Wars saga had to undergo before arriving on the silver screen.
While she appears only briefly in Return of the Jedi, the character of Mon Mothma quickly grew in importance. She became one of the few returning characters of the original trilogy who would appear in the prequels (Revenge of the Sith). Expanding further on her character, she even appeared quite a few times in Star Wars: The Clone Wars. Let’s take a look how this senator turned leader of the Rebel Alliance came to our screen!
Costume & Casting
With the big attack on the second Death Star, which in various early script drafts was accompanied by another Death Star and floating above Coruscant, it was clear that the Rebel Alliance would be expanded big time from what we saw previously, creating the need for military leaders like General Madine, Admiral Ackbar, and Mon Mothma. The first time Mon Mothma appeared in the scripts was in the revised rough draft from June 12, 1981. In a newly-added scene, she was to make an inspirational speech in a war room of a Rebel frigate while “a pale blue non-human creature” named Ackbar would give a strategy briefing. The second draft from September 21, 1981, would include Mothma bringing news that the Emperor himself is onboard the disarmed Death Star. With the second draft in hand and production already well underway, attention turned to the concept of Mothma, and it was costume designer Nilo Rodis-Jamero who would make the first sketch. With Ralph McQuarrie doing less work for Return of the Jedi, he made no sketches for Mon Mothma but would draw her later in a piece called “Rebels Plot Attack,” which is often confused as an actual painting from the production of the movie. This piece was made for the Return of the Jedi portfolio, but ended up not being included. It saw a later release as a giclée called “Plan of Attack.” While there were previous casting interviews, with Kate Harper (who would appear in the 1989 Batman movie as an anchorwoman) being the first to audition for the role of Mon Mothma, it was on December 15, 1982, that Caroline Blakiston had her turn. It was noted that she could cry on command if needed, and ended up winning the role.
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