I'm reading through Lawrence Stern's
Stage Management (8th edition) and just yesterday read the section on "Playwright at Work" (pages 154-155):
What happens when the playwright is present? The play still exists in only one place prior to the curtain's rise-in the director's mind. I know this sounds improbable, and I've never met a playwright who could accept it. But once the playwright commits his or her work to paper, and the producer turns it over to the director, the play is a concept in the director's mind.
...
[t]he playwright should go to the director privately to work on changes and should not communicate directly with the cast. The director should also suggest changes to the playwright privately.
It seems that the director should have final say, with a major reason being the cast should only be getting direction from the director. They shouldn't be getting guidance from the producer, playwright, stage manager, or other actors.