Well, you had to be there. And King says the line perfectly.
Anyway, the point is that our good friend Sam doesn't get such menacing lines as this, which is a pity. But his role of Hotel Manager Gerald Olin (which, if I remember correctly, was practically nonexistent in the short story) allows him to give dire warnings to John Cusack's skeptical writer Mike Eslin:
Mike Enslin: [Olin gives Enslin the room key] "Most hotels have switched to magnetics. An actual key. That's a nice touch, it's antiquey."
Gerald Olin: "We have magnetic cards also, but electronics don't seem to work in 1408. Hope you don't have a pacemaker."
Mike Enslin: [into his tape recorder] "General manager claims that the phantom in room interferes..."
Gerald Olin: "I have *never* used the word 'phantom'."
Mike Enslin: "Oh, I'm sorry. Uh, spirit? Specter?"
Gerald Olin: "No, you misunderstand. Whatever's in 1408 is nothing like that."
Mike Enslin: "Then what is it?"
Gerald Olin: "It's an evil fucking room."