You guys should really check out the original script to The Empire Strikes Back. It pretty much disproves the whole “planned out in advance” thing, and you can tell that they had planned for the story to go in a different direction. Most importantly the big twist is not there. It was a fun read though, I enjoyed it.
http://scyfilove.com/wp-content/uploads/2010/05/Star-Wars-The-Empire-Strikes-Back-Brackett-Draft.pdf
I’ve read that before and it really is fantastic. Really provides a lot of context for the screenplay we got.