Pedantic
Legend
I don't think this is quite capturing it. Focusing on backstory and "character arc" is still putting the emphasis on events and plot. The thing I was trying to drive at with the discussion of the primacy of character over situation is that this playstyle exalts characters as independent of the things that happen to them. A character is distinct and discrete from the stuff happening around them, and complete with or without it. You can imagine that these characters would be interesting and engaging to their players even without a campaign to exist in. They could tell you about them without reference to the events of any given game.I think in both cases (but especially story now), turning the backstory up to eleven makes it hard to actually play the character, for the same reason it's hard to play in a setting with too much detail. If everything is already decided, anything I or anyone else does at the table risks contradicting the truth that is already established. This is why I look at these characters with detailed backstories and carefully planned arcs and wonder whether they are actually fun to play, or even possible to play at all, as written.
When I look at the example of freeform roleplay given by @The-Magic-Sword , it seems to me that you couldn't actually plan detailed character arcs and be sure everything would work the way you planned. The same goes for backstory -- what if another player suddenly implies your two characters used to have a relationship?
I drew that line earlier between "expressive" vs. "growth" orientated characters, but they still have more in common than apart. An expressive character doesn't really change at all; any way such a character is different by the end of a campaign will be framed as self-discovery or explication of established traits. Selina is a thief->Selina is a thief with a heart of gold->Selina is a thief, but won't steal things under these circumstances, etc. A "growth" oriented character has places where change is called for in their plan, and might have a sequence of triggers/events they're looking to react to, but that plan is still encoded in the character from the outset. The exact shape of the tree is uncertain, but it is an oak seed and could never be anything else.
With that in mind, "detailed character arc" is a bit of an overstatement. These characters don't tend to need one. They're looking for a few significant events/details to happen in a story, or for the specific questions they've laid out in their past to be asked and resolved, but they aren't generally concerned with the nature of that resolution.
Your point about "what if someone implied these characters were in a relationship" resolves in two ways. First case, the question could be a violation of some tenet that defines the characters in question (i.e. one of them has "is an aromantic loner" as a defining trait), and thus would either never be asked because everyone involves understands the norms, or would fall into a discussion of out of game courtesies/preferences. Second case, the question isn't definitional to either character's traits, and that becomes another situation the character could be placed in. It might be a place they could show off some character traits or expressions, but it doesn't rise to the level defining the character.
The point I'm trying to express is that there is no outside event, outside of those contained/planned for in a "growth" type character from the outset, that can change the character (or, alternately, any trait that is changed by outside events was never definitional to the character to begin with). The imposition of something of "import" on the character is a violation of the gameplay contract.