I confess I only read half the thread... LOL But I wanted to throw in an idea. I'm quite new to this, but sometimes it helps, as I'm still fresh from reading about what underlies the theory.
If you go back to the concept of the storymind, there are 4 throughlines because there are 4 perspectives we usually consider when we set about making any kind of decision :
1/ what would happen in the world, and what would it look like from the outside, if people/anybody did X (OS)
2/ how I'd feel about doing X or why I would want to do it (MC)
3/ how you would react if I did X (IC)
4/ what would happen to our relationship if I did X (RS).
I think the subjective/objective binary often blinds us to these last two perspectives, which is perhaps why we have such a hard time explaining or formulating them. Yet, I believe we subconsciously take the 'you' and 'we' perspectives into account much more often than the 'they' one, at least when it comes to all our little everyday decisions (especially in these days of confinement).
Should I make dinner? There won't be consequences for the world at large, and I don't think people who don't know me will care one way or the other. Objectively (ie in the OS perspective), there's not much to help me make a decision. Subjectively, though, maybe I'm hungry (or not). Maybe I really want to try this new recipe (or maybe I hate cooking and am really terrible at it). Whatever. That's the MC perspective.
If I live alone, that can be enough. But most of the time, it's not. So let's bring you (IC) into the equation. Say if I don't make dinner, you don't eat. That may yet influence my decision. But not so fast! Where's our relationship (RS) at? It makes a huge difference whether you're my young child, and I feel responsible for feeding you, or whether you're my roommate. And it makes a second big difference if we (roommates) agreed to take turns preparing dinner and it's my turn (or not), or if you keep taking advantage of my cooking skills without ever pitching in.
That's the RS throughline for me. What your relationship is like, and how it may evolve according to the events that unfold. We almost never think about it, because it's so obvious, but we constantly adjust our behaviour not only based on general human principles (OS), on our own feelings, fears and ideas (MC), on how other people may react to us (IC), but also on who we're interacting with and how we conceive of our relationship with them (RS).
Concretely, I think the RS is created by you + me + context. And part of the context is shown by the interaction itself (that's how it's "alive" within the story). Say, if I don't make dinner, you get angry (IC). But the way I will then react to your anger owes both to who I am (MC) and to what our relationship is (RS). Like, maybe I tend to cower before angry people or, on the contrary, anger directed at me tends to make me angry as well. That would be my general disposition towards unspecified people (that would be IC + MC without RS). But who 'we' are can put a serious spin on this.
Maybe I brush it off because I don't even like you, and you don't hold power over me. Or maybe it scares me, because I've seen you get violent when you're angry, and I'm at your mercy. Or maybe I feel frustrated, because you get angry a lot. Or maybe I'm surprised and suspect something is off, because you never got angry at me before.
All these interactions, by themselves, say a lot about the relationship. But I can still imagine plenty of completely different situations to each scenario... Like, is the violent guy the MC's captor, or her husband? Part of the context is also given by exposition. Now, exposition is usually rather static, but I guess even it doesn't have to be... Married people can divorce, or someone's identity could be revealed to be other than was initially believed. (Having said that, just like for characters, you can depict a relationship that doesn't change, that endures in spite of all the obstacles thrown its way. That's a story of its own.)
Playing with the RS is a good way to isolate it. Let's say instead of coworkers, they're lovers. Let's say instead of mistrusting each other because of past deeds, they've just been introduced to each other. If not much changes in the story when you do that, then you're probably not exploiting the RS throughline to its full potential. Remember, it's supposed to be a fundamental perspective in an argument. It's not just extra flavouring. And all this 'advice', of course, is completely directed at me too! Please let me know what you think, I am completely open to reconsider or complicate my current view.