
When Darth Vader (David Prowse) revealed his secret to Luke, Hamill was hanging onto a pinnacle above mattresses placed on cardboard boxes about 30 feet off the ground.

Phil Tippett and Jon Berg stop-motion animate a shot with all three walkers (the background walkers are cutouts). Originally the plan had been to photograph the walkers against four-by-five Ektachrome transparencies shot in Norway; when these didn’t work as planned, matte artist Mike Pangrazio copied select Ektachromes onto large scenic backings.

more here
http://www.vanityfair.com/hollywood/features/2010/10/the-making-of-the-empire-strikes-back-201010?currentPage=all&ref=nf

