OP said "Sure, but the story isn't going to be like a movie, for most of us." I'm just pointing out that life is, in fact, experientially like a movie just maybe not the feel good story of the year we'd like it to be
Do you not see the logical fallacy here? If the generalized "you" - that is to say, the randomly sampled human - is merely a side character to someone else and essentially "unimportant", then that implies by induction that we are all unimportant which is surely not the case, as this distinction implies and indeed requires a hierarchy of important "main" characters. So who is a main character?
The real world is purely emergent so thinking you are a main character is just a fallacy, an absurdism.
What you seem to be pushing back on is that when someone describes themself as a side character it implies that they still buy into "narrative drive" and they just think poorly of themselves.
I think while one could argue that there are no main or side characters the simple fact is you can look at the friendship paradox to inform how you think about the problem.
Let's forget narrative in the manufactured sense and talk about what scale of perception a person referring to themselves as a "side character" may be implicitly using as a reference. Does the universe revolve around you to any impactful degree and the answer is that it doesn't at all, one asteroid and we are all done, we are here by happenstance, on any cosmic scale or timeline we currently hardly exist.
From an experiential human society scale some people function as higher power nodes than others and there is a good chance you are a lower powered node than the people you know, meaning each person is their self probably the least significant person they know.
I'd argue that internalizing the friendship paradox and what we know about social graphs could reasonably lead someone to refer to themselves
colloquially as a side character, "pretty much everyone I know is probably more important than me." is a fair self assessment for people to make.
