@GalladeGuy
Is an acting technique that consists on extracting emotions from one's self experiences in order to perform fictional or not fictional characters. It's actually a weird thing to explain from the outside... But let me put it in a way that's easier to understand:
There's an idiom out there "Fake it till you make it". That's kinda how acting works. Except with method acting you don't aspire to fake something, but to
become said thing. Through introspection and analysis of human behaviour one aspires to create the past and the future of said characters until the audience finds them real in the present of the narrative.
People here aren't mentioning the MC from Act Age for nothing. What she did was watch a ton of movies and recall the emotions she felt while watching them while performing but sometimes that isn't enough and that's where you hear all this crazy stories about actors going on trips to the forest to hunt alone in order to perform a character that does that same thing.
@osqmu
It seems that (And this is pretty cool) the star eyed thing is actually a metaphor for... something probably related to the psychology of the character. And not related to a condition on Ai's genes... As a matter of fact, when Ai is dying the stars in her eyes start getting smaller and smaller until she eventually... fades away. (Yes, I went and re-read that chapter just for this and now I have goosegumps)
So yeah, now we gotta write theories beyond "Aqua and Ruby are the children of
the love of a star (Hoshi no Ai) and that's why they have one star each"