In acting, it can work the opposite way to how you describe of course Mike.
Years ago I watched a television production of Théresé Raquin featuring the great Brian Cox.
Cox played it in a straight 'English' accent up to an extremely dramatic scene towards the end, where the role appeared to take him over and he roared his way though his lines in broad Scots - superb - still makes the hairs on the back of the neck bristle to think of it.
Jim Carroll