Not sure if that whole thing was supposed to imply that the "bystander" is Tsubame's actual "inner self" or not, I guess we'll see.
I understood that part as, Tsubame was a lot more expressive than the director expected from her, showing her "inner self", but she herself wants to completely erase her own person and become the character. I might be misremembering some things a bit, but we have been shown that Tsubame has this weird idea about not being "real" and she basically puts on a persona during her daily life as well. Thus, telling her that she herself shines through during her performance, wouldn't be what Tsubame wants to hear.
I find your reading interesting as well, though, and I believe it would still fit with the whole "real" theme. Is Tsubame a "real" person, or just a bystander watching others' lives? If she's striving to become the first —which I am not sure is the case— she wouldn't want to to be told that the role of the bystander fits her.
It's sad that this one probably won't get the time to develop as the author wanted it to. It has some interesting themes that I would have liked to see fully explored.