Its fairly common knowledge that Marquand was, for all intents and purposes, Lucas' sock. For ESB he gave Kirshner control over everything and Lucas only visited the set three days. The movie was a management disaster and the final product dissapointed him. So on ROTJ, he found a director that was young and moldable, that wouldn't impose his own ideals as much, gave him hardly any control over the film, and then Lucas was on set every day, setting up shots and directing actors and even doing second unit directing. Lucas was, essentially, a co-director, which is why ROTJ feels so much like a Lucas-film.