I think one of the things I really like about theater is the partiality of its artifice: as soon as you're backstage--even onstage--it's evident where all the secrets come from. That book you're saying is the bible is really a math textbook from the '20s, those cigarettes have jelly in the center, and none of the windows open in the palace. Film doesn't get away with that kind of thing. There is a high premium on "realism" because, on one level, it really is necessary.
I've been worrying about the contemporary relationship between film and theater for a while now, and I'll post the results as soon as there are any. Until then, here's Jerzy Grotowski on the stakes of the debate: "In our age when all languages are confused as in the Tower of Babel, when all aesthetical genres intermingle, death threatens the theater as film and television encroach upon its domain. This makes us examine the nature of theater, how it differs from the other art forms, and what it is that makes it irreplaceable." (From "The Theatre's New Testament" in Towards a Poor Theatre)