We generally add points and comma to emphasize parts of the text, slow down, stop the reading... Try reading the text out loud without the syntax highlighting, then try with. See? It doesn't work, you're going to emphasize and stop on higlighted words. What would have worked would have been to highlight everything and split the highlight according to punctuation, not words.
As someone else pointed out, different colors for different protagonist talking would be neat as well. But there is not much you can't do without deforming the original intent of the writer.
Another thing that could be interesting would be to give these tools to the writer instead of highlighting the text automatically, but then as someone else pointed out, these tools already exists and are rarely used because of the noise they add (bold, italic, underlined). There are also quotes, quads, uppercase, ... There are many ways to help the reader follow the text.
As someone else pointed out, different colors for different protagonist talking would be neat as well. But there is not much you can't do without deforming the original intent of the writer.
Another thing that could be interesting would be to give these tools to the writer instead of highlighting the text automatically, but then as someone else pointed out, these tools already exists and are rarely used because of the noise they add (bold, italic, underlined). There are also quotes, quads, uppercase, ... There are many ways to help the reader follow the text.