The Expanded Universe is actually older than the Star Wars movies themselves. It started in 1977 when the novelization of Star Wars hit book stands before the movie was released in theatres. The novelization was ghost written by Alan Dean Foster. Since Foster was writing the book before the final cuts had been made to it, there were several scenes that he wrote for the book that would not appear in the film thus expanding the Star Wars Universe.
The early years of the Expanded Universe where sporadic and unrefined because there was very little for the authors to go on. The first EU novel to not be an adaptation of the films, Splinter of the Minds Eye by Alan Dean Foster, drew inspiration largely from one of the original drafts of Star Wars. Many of the books and comics from the 80s were contradictory and made many analogies to our reality. It wasn't until West End Games started publishing material for the Star Wars roleplaying game that the EU really began to be fleshed out.
The EU was by in large not very successful in its early years. Many readers were turned off by the apparent lack of consistency of it all. It wasn't until Timothy Zahn wrote Heir to the Empire in the early 90s that it really began to catch on. Heir to the Empire was the first book in Zahn's Thrawn Trilogy and it sparked a revolution in Star Wars literature. After Zahn followed numerous Star Wars novels, most being well received.
It seems that elements of the expanded universe influenced George Lucas in the writing of the Star War prequels, at least insofar as knowledge of the EU helps in understanding the prequels. For example, in episodes 4-6, the concept of Darth Vader being "The Lord of the Sith", or the emperor being called "Palpatine" are not really explained on screen. The fan fiction, or EU concepts that were developed however seem to have been adopted by Lucas in his newer movies. In fact in Attack of the Clones there is a Jedi character, Aayla Secura, who was made for use in the Star Wars comic book. Lucas liked the way the character looked so much that he had a costume of the character made for the movie.
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