Called again to explain the very important difference between Main Character and Protagonist (as defined by the Dramatica theory of story - the most comprehensive understanding of story to date), I reply:
Main Character - viewpoint character which INCLUDES personal baggage tied to the story’s central inequity
Protagonist - character who PURSUES the successful resolution of the story’s goal regardless of an intimate look into any personal baggage
Sometimes one and two are the same (most American films), but not always.
Neither “The Ishmael” nor the “Supporting Protagonist” as defined by this site [the TV Tropes site linked above] includes that key component of personal baggage. Audience Surrogate sounds closer to the Main Character concept described above, but again I can’t find reference here to the requirement that there be personal issues pertinent to the story at hand.
That’s the key.
In The Shawshank Redemption, Red not only provides the narration he gives us an intimate look at what it is like to go with the system. He is an institutionalized man and shows up to each parole board meeting with hat in hand, saying what he thinks he should say.
This is the same problem everyone else in the story has to deal with. From the fresh fish not following orders to shut up to good ‘ol Tommy doing what he can to help the Warden out, everyone in Shawshank suffers from the effects of supporting the powers that be.
But we’re seeing their stories from afar, from a third-person objective perspective. Andy, who clearly is the one driving the story’s successful resolution and thus is the Protagonist, spends an inordinate amount of time in the hole, yet we never get to feel what that is like. He goes in, he comes out and suddenly he’s a different person. We are not him.
Red’s journey, however, we see from a very personal very first-person perspective. Red’s surprise is our surprise. We even walk through the jail doors into his parole board meetings! We’re with him every step of the way.
The dissonance created between the two perspectives gives the story meaning and is why we feel compelled to watch it over and over again. In short, it is giving us something we can’t get in real life--a simultaneous look inside and outside of a problem.
Same with To Kill A Mockingbird. Scout is more than simply the narrator. Yes, Atticus is a “man of action”, but that is his role as Protagonist--trying to resolve the issues of injustice that sit at the center of the story.
Scout, though, has her own personal issues revolving around justice, particularly within her relationship with the boogie man, Boo Radley. Sitting in her shoes we get a very intimate look at what it is like to be prejudice, or unjust, towards another. We get to see it from within through her, and from without through Atticus. Again, that dissonance between the objective and subjective gives us as an Audience the meaning we can’t find in real life.
That’s what makes stories so great.
Two other articles that elaborate on this idea: The Confusion Between Main Character and Protagonist and Redefining Protagonist and Main CharacterConcepts covered: Main Character Throughline & Protagonist.