Her response here touches on so much that we've tackled on this site - particularly, as the title of this post states, this burden of representation that some of us feel black actors should carry. The question of "obligation."
We've argued the matter quite intensely on Shadow And Act, as long time readers will know, but never really reaching a consensus; not that we have to. It's an issue that's long been debated before all of us (including old man Sergio) were born, and I'm sure it'll continue to be just as polarizing.
Lee Bailey at eurweb caught up with Viola Davis, who's currently doing press for the upcoming film, The Help, and I think this is as emphatic a response she's given thus far (note, some of us have seen the film, but some of us aren't allowed to talk about it just yet; kinda like Fight Club):