In this short film, special effects artist Dennis Muren explains how he and his team animated the AT-AT Walkers from The Empire Strikes Back. Have a look—this is just under two minutes:

The Star Wars YouTube channel elaborates:

Dennis Muren looks back on how Industrial Light & Magic animated the AT-AT walkers from The Empire Strikes Back's Battle of Hoth sequence—and the inspiration for the techniques used.

Originally, Muren and his team were unsure of how they would bring the AT-ATs to life. The first idea was to build an actual robot that could move by itself, but that was deemed too complicated and costly. Instead, Muren pushed for stop-motion, citing the influence of King Kong and the realization that the staccato look of stop-motion would be appropriate for machines. Models were manipulated a frame at a time, animated in front of painted backgrounds instead of blue screen, with baking soda was used in place of snow.

It was shot at 24 frames per second, resulting in about 5 seconds of footage per day of work. For explosions, high speed photography was used, and cutouts were used for background walkers.

One of the early ideas was to build an actual robot version that would walk on its own, but that would prove too costly and complicated. Muren, whose background was in stop-motion animation, pushed to have the sequence done using that technique—since the AT-ATs were machines anyway, the staccato look of stop-motion would be appropriate. So stop-motion models were built and manipulated in front of paintings, as opposed to blue screen, and baking soda was used for the snowy landscape. The set itself had trap doors so that animators could pop up, animate the model, go back down, and shoot a frame of film. Photo cutouts were used for walkers in the background, and smaller models were created to convey a sense of scale and depth in the shots.