Melanie has a recent blog post, The Dramatic Triangle, which is fascinating -- a different way of explaining some well-known Dramatica concepts.
One section deals specifically with the topic of this thread, the Influence Character who is also the protagonist:
A variation of this is for the protagonist to also be the influence character, rather than the main character. Then, he would be fighting against the antagonist in the overall story, but he would also be trying to change the beliefs of the main character, who is the third corner of the triangle.
In this arrangement, we would not be seeing things through the eyes of the protagonist, but through the eyes of the main character. We stand in the shoes of the main character and watch the protagonist/influence character, but be personally challenged to change our beliefs by him.