Actors are called upon to do a wide range of accents, which is more challenging than many people appreciate. Even a highly skilled actor such as Tom Hanks can drop an adopted accent amidst the demands of playing a complex scene (see, e.g., his intermittent Bostonian voicing in Catch Me if You Can). Other actors do an accent so poorly that you pray they will drop it so that you never have to hear it again (e.g., Kevin Costner’s English accent in Robin Hood).
One of the best accents I have heard in recent years was Matt Damon’s masterful take on a white South African’s voicing in Invictus. Ever behind on movies, I only saw True Grit last night, on the plane home from London. Damon pulled off a dead-on impression of a man with a serious tongue injury. I have worked with patients with such wounds, and to my ear he was simply spot-on. Bravo.