Eygon is a good example of great unwritten writing in games (games can't force the player into a story, the player creates the story, but games can provide options for the player to tell their own story. These options are what I call unwritten writing, because as a developer you don't write dialogue or cutscenes for them):
Yesterday I had a bad day, so I decided to play Darksouls.
Eygon pissed me off and I didn't know if I could get his weapons later in the game, so I've killed him.
Then I had a giant club to squash my enemies, which always feels good, especially in darksouls.
So despite taking away from the written story (killing Eygon), I got great story out of it (squashing enemies with a club on a bad day).