I've been reading through some articles over at Jim Hull's Story Fanatic site and he has some interesting thoughts on the differences between a "Protagonist' and a "Main Character." For more information, check out these pieces:
The True Definition of a Protagonist
Redefining Protagonist and Main Character
Jim also raised a question about whether or not Nic was the true protagonist of "The Kids Are All Right" since she doesn't seem to go through a significant change by the end of the story, which I thought was an interesting point. I do think she changes, but I agree that the change is pretty subtle, which is simply an observation and not a criticism at all since I liked the ending. Within days, a friend of mine named Zenas asked the same question about how Olive could be the protagonist of "Little Miss Sunshine" when she isn't the one who changes in the end. This has inspired me to add a section in my screenplay analyses from now on about which character undergoes this kind of change and how that is demonstrated in the script.
Here are my thoughts so far: A protagonist should struggle to overcome an internal flaw (or to fill an internal need) in order to accomplish an external goal. The protagonist's character arc is represented by how much progress she makes in overcoming her internal flaw, so it seems like a good script should clearly demonstrate the amount of internal change the protagonist has achieved. From now on, that specific question will be in my script analysis template, because I'm not convinced yet that it's a 100% requirement for the protagonist to be the character who demonstrates that internal change. It could be this "Main Character" that Jim refers to in his articles, which would certainly be reflected in the amount of change that Olive's step-dad Richard goes through over the course of "Little Miss Sunshine." Fun stuff to think about!