So I've gone back and reviewed the earlier cutscenes more closely, and... a few of them read completely differently once you know the true nature of Noct's calling.
Let's start with the conversation between Noct and Cor in the first royal tomb:
Cor: At last, Your Highness.
Noct: Yeah, wanna tell me what I'm here for?
Cor: The power of kings, passed from the old to the new through the bonding of souls. One such soul lies before you. To claim your forebears' power is your birthright and duty as king.
Noct: My duty as king of what?
Cor: Now is not the time to question your calling. A king is sworn to protect his people.
Noct: And yet he chose to protect only one prince. Was that his calling? Forsake the masses to spare his own son?
Cor: How long will you remain the protected? The king entrusted the role of protector to you.
Noct: "Entrusted" it to me? Then why didn't he tell me that? Why did he stand there smiling as I left? Why-- Why did he lie to me?
Cor: That day, he didn't want you to remember him as the king. In what time you had left, he wanted to be your father. He always had faith in you, that when the time came, you would ascend for the sake of your people.
Noct: Guess he left me no choice.
Cor: The power of kings goes with you, Your Majesty.
Of particular interest is Noct asking, "My duty as king of
what?" and Cor responding, "Now is not the time to question your calling."
The first time around, Noct's line seemed like a sarcastic reference to the fact that there wasn't a kingdom for him to rule anymore after the fall of Insomnia. But that interpretation is sort of at odds with the fact that every other time a "calling" is mentioned, it's in reference to a cosmic duty rather than an earthly one. As such, Noct's line can be interpreted as him basically calling out Cor for pressuring him into doing King of Light stuff instead of just King of Lucis stuff.
When Noct asks, "Was that his calling? Forsake the masses to spare his own son?" it's similarly interesting. If "calling" is taken to refer to Regis' cosmic duty, this is basically Noct asking, "Did the gods demand that my father sacrifice an entire city to save me?"
And then there's Cor's doozy of a line, "He always had faith in you, that when the time came, you would ascend for the sake of your people." This is a clear reference to Noct's ascension as King of Light as opposed to a simple coronation, IMO (which is why Noct's response is so negative). It's not clear who apart from Regis and Luna understood that Noct would have to die, but Cor at least seemed to know that Noct would be required to undergo an ascension of some sort.
Moving on, there's a conversation with Cid that takes new import in light of the ending:
Cid: Ain't nothin' nobody could've done to stop what happened.
Noct: Yeah, I realize that.
Cid: But you need to realize just what you mean to the boys by your side.
Noct: I do.
Cid: Even if they can't solve your problems, you can't hide what's goin' on from 'em. It hurts like hell. Remember -- those ain't your bodyguards, they're your brothers. Trust in 'em. Always.
In this case, it's less clear whether Cid is talking about Noct specifically or just offering Noct a lesson based on his own experience with Regis, but "[Y]ou need to realize just what you mean to the boys by your side" and "Even if they can't solve your problems, you can't hide what's goin' on from 'em" basically prefigure the camp scene at the end of the game, where (unlike Regis) Noct
does confide in his friends what's going to happen to him, even though there's nothing they can do to stop it.
I do sort of suspect that Cid
was talking about Noct specifically, though, if only because Noct had already been ambivalent about his duties in ways his friends didn't seem to understand.
Switching over to the villains, there's Ravus' almost impenetrable interaction with Noct that makes quite a bit more sense in retrospect:
Ravus: You received the Storm's blessing. And yet, you know nothing of the consequences.
Gladio: Watch it.
Ravus: Be still. All of you.
Prompto: Not good...
Ravus: Heir to a crown befitting no other... Witness his splendor and glory. All hail the Chosen King.
Noct: Awful high and mighty for an imperial rat, serving the enemy to hunt down Luna!
Ravus: I do not serve. I command!
Ravus: The king's sworn shield.
Gladio: You better believe it.
Ravus: A weak shield protects naught.
Noct: Hey! Wanna go? Let's do it.
Ravus: Should the Chosen fall, that too is fate.
Ravus knows that what Luna is doing for Noct in seeking the blessings of the Astrals will kill her. He likely knows that Noct is fated to die as well. And he's very bitter that this "weak," ignorant kid is critically important to the fate of the world.
It also seems implied that Ravus is willing to condemn the world in order to remove Luna's need to make further Covenants, both here ("Should the Chosen fall, that too is fate") and in Ardyn's conversation with him in Altissia:
Ardyn: The wedding day arrives, but alas, without the bride. Of course, we've come for the Hydraean, and you've gone to such lengths to prepare.
Ravus: Merely my duty.
Ardyn: Ah, but for an outsider to lead the imperial army must be a battle in and of itself. You've spoken to Lady Lunafreya?
Ravus: No.
Ardyn: That obstinate secretary, standing in the way, while you rush off to slay the Hydraean for your poor sister's sake. I know the price of the covenant.
Ardyn's lines reflect both his own knowledge that Luna will die from forging the Covenant with Leviathan and his suspicion that Ravus is trying to kill Leviathan before that happens to save his sister. Of course, we know from later flashbacks that his motivations changed before he could attempt anything like that and he ended up providing support to Luna in her plan to save the world instead of risking the world to save her, but I think we're intended to believe that that would have been true at some point. Depending on when said flashback happens, Ravus might also be lying to Ardyn about not having spoken to Luna; if the flashback happened afterwards, seeing that it was already too late to save Luna and how much she'd given up already might have been the trigger for his change.
(On a different note, Ardyn also hints at his own nature, which is really obvious in retrospect -- "Ah, but for an outsider to lead the imperial army must be a battle in and of itself.")
Anyway, speaking of Luna, her final conversation with Noct takes on some rather different implications in light of Noct's own fate:
Luna: Noctis.
Noct: Luna?
Luna: So, you found your way here.
Noct: And you found me.
Luna: A chance to see you once more... Who would have thought?
Noct: What do you mean? Why wouldn't you see me?
Luna: Because my prayers have been answered. My calling fulfilled.
Noct: But... that doesn't have to come between us.
Luna: You are the One, Noctis. The Stars shine for you now. That which is yours by right shall be restored to you. Do you remember the flowers of Tenebrae? It seems so long ago. You'll find they await you still, blooming from hill to vale.
Noct: Will you be there?
Luna: Would that I could join you... but this moment will have to be enough.
Noct: It's not right. All I... All I wanted... was to save you.
Luna: When the world falls down around you, and hope is lost, when you find yourself alone, amid a lightless place, look to the distance. Know that I am there, and that I watch over you always. Farewell, dear Noctis.
Of particular interest is that Luna's response to Noct asking, "Why wouldn't you see me?" is "Because my prayers have been answered. My calling fulfilled," and her redirection to Noct's own calling when he replies, "But... that doesn't have to come between us." She doesn't specifically say that the fulfillment of a calling is tantamount to death, but the implication is there, especially since it's pretty clear that she isn't referring to the lands of Lucis when she says, "That which is yours by right shall be restored to you."
There's a line of Noct's here that's interesting in light of his fate, too -- "It's not right. All I... All I wanted... was to save you." While it's unclear how much he knew about what would inevitably be expected of him, there are many indications that he knew that the consequences of being King weren't going to be pleasant. And in light of that, this line seems like he's saying, "I know there's nothing I can do about my fate, but I really wanted to spare you from it." =(
Another interesting line of Luna's came shortly before this:
Luna: When the prophecy is fulfilled, all in thrall to darkness shall know peace.
There's some question about what happens to people like Ravus -- who were turned into daemons -- in the afterlife, but I think this line can be taken as assurance that they're not just left to suffer.
Anyway, after Luna's death, Noct's duty becomes a much bigger focus:
Gladio: How's that ring fit ya? You'd rather carry it around then wear it? She gave her life so you could do your duty, not so you could sit around feeling sorry for yourself.
Noct: You don't think I know that?
Gladio: You don't! Ignis took one for you too, and for what?!
Ignis: Enough, Gladio.
Gladio: You think you're a king, but you're a coward.
Noct: Shut up!
Prompto: Don't do this--!
Noct: I get it, alright?! I get it!
Of particular note is Gladio saying, "You think you're a king, but you're a coward," because that can't just be Gladio accusing Noct of being too sentimental about the lives lost for the sake of the ring. He's saying Noct is scared of his duty and that he's feeling sorry for himself because of
his own fate. And, while I don't think we're meant to agree that Noct is a coward for not being ready to die, I do think Gladio's supposed to be right about the real reason why Noct refuses to put on the ring.
The second conversation with Gladio on this subject includes another example of Noct calling people out for talking about Chosen One stuff without being explicit about it:
Gladio: Hold up. You sure you're ready for this? You got what it takes?
Noct: To do what?
Gladio: To face your ancestors and convince them to lend you their strength. Got a long road ahead. Can you see this through? To the end?
Noct: Can and will. Whether I like it or not, I've got a duty to fulfill -- as king.
Gladio: You're damn right you do. Then that means you'd better start taking this seriously.
Gladio starts by talking about the immediate task at hand, but then shifts over to talking about seeing things through "to the end." And Ignis mirrors this way of conceptualizing the party's shared duty later in Chapter 10:
Ignis: I would remain with you all. Til the very end.
And, by the time Noct sees the vision of Luna, he seems to have all but accepted that his duty will inevitably claim his life:
Noct: Luna... It's so hard... Guess it was hard for you, too. I'm sorry I couldn't be there for you.
So I think the way we're meant to take Noct's relationship with his duty is that he's always kind of avoided thinking about it because he knows it's going to be awful for him (there's some possibly-optional dialogue outside the throne room where he mentions how sad his dad always looked when he saw the paintings of the prophecy =( ), but Luna's death and the presence of the Ring forced him to confront it more directly. And then when he sees Bahamut, it's less a total shock and more a confirmation of the details of something he's always known was coming (which is probably why he doesn't even attempt to fight it and just looks like he's about to cry instead =( ).