At what point do you have to pay copyrights to the original author? I thought of a line a few days ago that perfectly describes my main character's drive, but it seemed oddly familiar, so I researched it and discovered that it's from The Inheritance Cycle by Christopher Paolini. Could I make it an in-universe thing that he read it in a book somewhere is where he got it from? C.P. had a reference to Doctor Who in the third book so I'm kinda lost on where copyright stretches to