I've seen a few instances of people tweeting as characters from books or tv shows, and mostly, they come across pretty contrived. Done well, however, and the potential for hilariousness is endless. Take HalfPintIngalls, for example, in which Laura Ingalls Wilder tweets from the past. I adore it. The way in which Mrs. Wilder uses her 19th Century sensibility to interect with the present is positively spot-on.