Pictures are written, acted, directed, photographed, edited, scored and all that. The screenwriter determines what scenes are in and what scenes are out; decides whether that bit of information is dramatized or just referred to; whether it takes place on or off screen. There are millions of decisions made by the screenwriter.
You have to understand that people feel threatened by a writer. It's very curious. He knows something they don't know. He knows how to write, and that's a subtle, disturbing quality he has. Some directors without even knowing it, resent the writer in the same way Bob Hope might resent the fact he ain't funny without twelve guys writing the jokes. The director knows the script he is carrying around on the set every day was written by someone, and that's just not something that all directors easily digest.
One of the tricks is to have the exposition conveyed in a scene of conflict, so that a character is forced to say things you want the audience to know - as, for example, if he is defending himself against somebody's attack, his words of defense seem Justified even though his words are actually expository words. Something appears to be happening, so the audience believes it is witnessing a scene (which it is), not listening to expository speeches. Humor is another way of getting exposition across.
Only grown-up men are scared of women.
But it is not at all unthinkable for anyone to tell a writer how to write. It comes with the territory.
