Also, please read this interview with Kasdan:
Extract that took my attention:
Q: When you came to write The Empire Strikes Back, had the decision already been made to make it darker than Star Wars, or was that your influence?
A: I think I influenced that, and George was open to it. Over the three Star Wars films, he saw a trajectory. The Empire Strikes Back was the second act and, traditionally, the second act is when things start to go bad. George had made his biggest decision when he hired Irvin Kershner to direct, even though Kershner and I were acting as his tools. When it came to the third act, The Return of the Jedi, which functions as the relief, he chose a different kind of director, Richard Marquand, whose world view was much sunnier than Kershner's.