At the end of The Empire Strikes Back, Darth Vader reveals that he is Luke Skywalker's father. It's one of the most famous scenes in cinema history, but the script used to film that scene wasn't the ACTUAL script and for most of the cast and crew, the first time they heard "I am your father" was at the first screening.
Some of the Irrational team are going to be in a similar situation when the game comes out, as they are yet to be told huge chunks of the story. Head of the team, Ken Levine, explains why he wants to keep them, and you, in the dark as much as humanly possible:
“I think I would rather err on telling too little than too much," he said to PSM3, "because you don’t want to ruin it for people. The way we develop, even most of the team here doesn’t know large chunks of the story. I want to preserve them, and observe them observing those parts of the story so I can see their reactions. Keeping people virgins to that content is really important because you never get that reaction from someone the second time.”
You can read the full interview, with a few spoilers, over at CVG.