Another thing to remember too, is that Jim pretty much gave everybody who worked for him a great deal of their own creative control: he may have had a vision for something, but he let others take that vision, and turn them into something special. In fact, so many of the characters we know and love today are the results of the individual performers taking them and breathing their own life into them... I really can't think of a whole lot of characters (save for maybe Big Bird and Oscar the Grouch), where Jim himself figured out all of the details of how he wanted them to behave, act, sound, etc.