Very interesting question.
An interesting possibility comes to mind. I didn't think about this before but do you think authors used a different kind of writing for the different characters in their book? If they did that they would have known that it wouldn't be seen by the reader, but perhaps they still unconsciously did that. Did you check manuscripts? For instance, lately Madame Bovary with all of Flaubert's annotations was put online: http://bovary.univ-rouen.fr/ . It would be interesting if he used a slightly different kind of handwriting when it was Madame Bovary who talked, and when it was Monsieur Bovary. (From a quick look it seems completely unreadable to me). I wonder if that is ever the case.
Also, there is the graphic novel, aka comic books. One of them, Watchmen
, I'm just reading now, and this issue comes up constantly as he uses many different techniques.