Sometimes, the players say things that get very close to the line. Usually this happens when the MC asks a leading question.
MC: "Nero, what do the slave traders use for barter?"
Player: "Oh man, those bleeps? They use human ears."
That's a case of the player authoring part of the world outside their character, however -- and this is critical -- they do it from within their character's experience and frame of reference. When Nero answers that question, he's telling something he knows about the world.