One of the most memorable lines from the Lord of the Rings movies is when Gandalf stands before the Balrog and says "You shall not pass."
EDIT: Actually, he says "you cannot pass" in the movie too. It's just commonly quoted as "you shall not pass" because he says that later.
He doesn't actually say that in the book.
In the Fellowship of the Ring, Gandalf blocks the Balrog saying:
You cannot pass....I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass.
What is the reason for the shift?