So, the "playacting" language here is kind of loaded, and, I think, dismisses a major point, which can be illustrated by one game - Ten Candles.
Ten Candles is a tragic horror RPG, and all the PCs WILL die. There is no question about this, no way to avoid it. No clever last minute saves. All the players enter into the game knowing this fact.
This gives us a different perspective on agency. You are going to die - you can resist and try to avoid, but it will happen. So, then, what other things can you have agency over? It turns out there's lots of things you still have control over - like, do you reconcile with your estranged parents? Do you die bravely, or cowered in a corner? Do you embrace your faith, or reject it? And so on - the game is determining not whether you end, but how you end.
Even if you are on a plot railroad, you still have agency over the character, who they are, what kind of person they choose to be, how they feel, and so on.