Hi Lulu,
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.

I think this is also a major question for music. You are trying to write down something extremely expressive in a form which is completely fixed. You can try a bit to indicate phrasing, speed and so on, but for the most part you are dependent on the accumulated force of the melody and rhythm pushing its way out, arranging itself (once played out loud) in a suggestive way. In writing, you have a similar trump card: the words said aloud. Typography can be used to control the pace of reading (suggesting line breaks, breaths...) and this small spark thrown into the tinderbox of speech can do a lot.
Emily Dickinson would be an example.

There was a popular underground book (which I haven't read) called House of Leaves, by Mark Z. Danielewski, which uses footnotes inverse pages I think and tricks like that. (Danielewski was mentioned by Brian Malone in post).

Poetry, since always,  took control on how words are spread on the page. Still, this is rarely used outside poetry (and the graphic novel).

But more to the point, you ask "How do you interject expression into a set typography?" This is a good question but I must also note that that is what authors do. They interject character with no regard to typography, simply by using words. Not all authors, but for many they see it as simply words with no physical existence. It is somewhat striking how many authors feel that way.