"Dan Gregoire, JAK Films’ visualization effects supervisor on Episodes II and III, writes about state of previsualization on the Star Wars prequels.
Previsualization allowed creator George Lucas to show others the movie in his head before production started. All images from Star Wars: Episode II — Attack of the Clones © Lucasfilm Ltd. & TM. All rights reserved.
Previsualization — also known as previs has, among others, three main purposes: to sell a concept and to save time and money. For the Star Wars prequels, previs is an essential tool that George Lucas has used to paint an accurate picture of what his final films will look like long before, and even during, production. For a director such as Lucas, previs helps answer questions, explore options and aide in getting his point across to the numerous people in the production pipeline, offering more creative and financial control.
Throughout the Star Wars movies previs has been used to help convey the complex worlds we see onscreen. On Episode IV — A New Hope, extensive use of WWII fighter footage was used to help realize the final Death Star Trench battle. For Episode V — The Empire Strikes Back, hand drawn animated panels that resembled black-and-white Saturday morning cartoons were used to explain the Empire’s Walker advance toward the Rebel base on Hoth. Taking it even further, small video cameras were used with miniatures for the speeder bike chase in Episode VI — Return of the Jedi. With each movie, Lucas continued to use the latest easily available technology to describe complex scenes before a lot of money was spent on the final effects shots."