Film accents

I saw the Departed tonight, and among other things I was thinking about accents. This movie is basically like Goodfellas in Boston, and every actor in the movie made a conscious effort to sound a little more Bostontonian with his or her accent, which begs the question, what about Matt Damon? He's actually from Boston so why did he have to change his accent at all. Maybe since Matt Damon hasn't lived in Boston, or has done a lot of other movies that required a lot of different accents, in a while he felt he needed to overcompensate? The last movie I saw before this was All the King's Men and everyone in that movie really got into their Cajun accents, except for Patrticia Clarkson who just talked like she usually does. I looked it up, and Patricia Clarkson is from New Orleans, so she was smart to save herself the effort and cost of a dialect coach. It also means that she really is the standard by which all of the stars should've measured themselves in preparing for the role.