Word of Dante: Several details of the story have been used in so many stage and screen adaptions that it's surprising to learn that they weren't in Dickens' original. Belle is often referred to as Fezziwig's daughter, when no such detail exists in the book (in fact, many adaptions give her name as "Isabelle," whereas in the book she's simply called "Belle"). Also, the reason for Scrooge's hostility toward is nephew is never clearly spelled out, though most adaptions just assume it was because his mother died giving birth to him. Likewise, the reason that Scrooge's father is cold to him is never spelled out, but is often given a similar Freudian Excuse.
Word Of God: At the beginning Charles Dickens speaks directly to the reader to impress upon them that Jacob Marley was dead to begin with. He explains this one fact is absolutely crucial to the story, and therefore warrants extensive Word Of God confirmation, from death certificate to door-nail.