The November 9 New Yorker has a fascinating piece by Alec Wilkinson: “Talk This Way: The man who makes Hollywood sound right”, about dialect coach to the stars Tim Monich. (An abbreviated version is available on-line here; the full version is only available to subscribers.)
Monich is versatile: he taught Donald Sutherland, a Canadian,
to speak like a South African in “A Dry White Season,” then like an Englishman, a wealthy New Yorker, a New Englander, a Kansan, a Georgian, an Oregonian, a North Carolinian, a Mississippian, a Michigander, a Minnesotan, and a member of the Polish politburo
In fact, what he aims to teach is not some generic accent, but an accent appropriate to a specific character (though of course no coach can teach someone to reproduce an accent perfect in every detail). So in teaching Hilary Swank to speak like Amelia Earhart, Monich took into consideration that she
was from Kansas but had gone to boarding school near Philadelphia, and so had elements of a period upper-class accent
Monich has a huge archive of “recordings of talkers whose speech represents a particular place, period, or social station.” And he goes around the world collecting additions to this archive.
There’s an interesting account of a coaching session with actor Gerard Butler, in which Wilkinson struggles with the task of representing details of pronunciation for New Yorker readers without using the technical terminology or transcription schemes of phonetics.