All we know is during the final month, which was the Crescent City portion (Endor exteriors) Marquand was sort of pushed aside with Lucas and David Tomblin grabbing the shots to get the thing done on time because Marquand was too slow. That may be why that material is visually the dullest in 6 movies. Also, Lucas directed the Vader unmasking on a closed set. Other than that, Frank Oz has stated that Marquand WAS the director, but with a deeply involved producer over his shoulder. You know, like EVERY movie. Yoda's death scene was Marquand's idea (Yoda was already dead in the script and appeared to Luke as a ghost), and Leia and Lando infiltrating the palace in disquise was also Marquand's idea. Lucas also referred to "Richard blocking the duel" while he scrambled to re-write Vader's dialogue, meaning Marquand directed the throne room scene, which is a pretty damn good scene. He also cast Ian McDiarmid, so his contribution ain't nothing.