Looking at what he accomplished, it would be insulting to call his pictures "concept art"; they were Star Wars. Ironically, McQuarrie didn't have a lot of faith in the film, but that didn't stop him from illustrating the coolest universe he could think up, and Lucas was sold from the start. He even said after shooting the trilogy, "When words could not convey my ideas, I could always point to one of Ralph's fabulous illustrations and say, 'Do it like this.'"
But surely McQuarrie was just bringing George Lucas' ideas to life, right? There was, after all, some semblance of a story on which he built these worlds. Well, to get an idea of how pivotal McQuarrie was, take the most iconic character from the trilogy: Darth Vader. In the original script, Vader was only described as wearing long black robes; it was McQuarrie who decided he probably needed some kind of breathing mask to survive the vacuum of space.
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"Oh, and is it OK if I make him look awesome, too?"
Without McQuarrie, there wouldn't be a Darth Vader helmet, or a Boba Fett suit, or Storm Trooper armor, which means you would have spent an entire decade of your childhood dressing up each Halloween as a ghost or a skeleton or some bullshit. Unless you did do that, in which case, you should watch Star Wars, it's really good.
McQuarrie didn't just work on Star Wars. He also designed the mother ships for both Close Encounters of the Third Kind and E.T. You can even see some of his original drawings in the movie Raiders of the Lost Ark when Indiana Jones shows everyone the devastation the ark is capable of inflicting.
The Bible wishes it was illustrated by McQuarrie.