Ishmael narrates Moby-Dick, just as Gulliver narrates his travels and John Watson narrates the Sherlock Holmes stories. In each case we can assume that all the information presented in the literary story is imparted to us by its fictional narrator.
But the filmed version of each story contains thousands of details that are apparent to us but clearly never observed directly by the narrator. Yet it’s still the narrator who’s ostensibly telling us the story. If the narrator isn’t supplying these details, then … who is?