There's a theory in literary criticism (the heresy of paraphrase) that the meaning of a poem can't be reduced to a simple declarative sentence.

From a Playboy interview:

BOB DYLAN I do know what my songs are about.

PLAYBOY: And what's that?

DYLAN: Oh, some are about four minutes; some are about five, and some, believe it or not, are about eleven or twelve.

The meaning of a movie is the entire movie.

Blake Snyder stated that the theme of a script should be stated on page 5. Now that would make a screenplay not only preachy but formulaic. And if the audience knew the theme after 5 minutes, what would be the purpose of watching the rest of the movie?

A more profitable approach is to phrase the theme as a question (one with at least two valid answers, not a loaded question). The goal of the story then is to answer or resolve that question, which it tends to do at the climax. The character choices determine the theme, not the reverse.

The writer MIGHT know the theme at any point in the writing / revising process, but the audience should not know the resolution until the end.