I've realized this scene is too much for the first chapter and makes more sense when they regroup at Marathon.
This scene starts off because Serinth gives Heyoke some words of encouragement, but they're all completely hallow. Something like "I know you're afraid, I am too." and then some other bullshit. Instead of encouraging Heyoke it ends up scaring him. He calls her out on it, saying she doesn't really mean it at all. She says no, she guess she doesn't, she thinks maybe she did at one point though. This leads to an awkward silence and the following conversation:
Heyoke (trailing off): What's...
Serinth (finishing for him): What's...it like?
Heyoke nods hesitantly.
Serinth (considering it as she continues to stare down the ravine): It's like being in a play. A play I've been rehearsing for so long that I know it all by heart. And all the other actors are always so enthusiastic to play their roles but... we can't quite seem to nail it. Somebody is always screwing up the little details. And I've been doing it for so long that I can't really remember what the play's about why I wanted to be part of it.
Serinth (cont'd, turning to him): Even here, right now as I'm saying these words to you. I know who you are. And I know what Serinth is supposed to say to drive the plot forward. But I don't know why she says it, or how it's supposed to make her feel.
Serinth (cont'd, almost apologetic): I have this concept of Serinth. This sort of high-level abstraction. Her history, her relationships, her strengths and weaknesses. I know all her dialogue, and I know her role in the story. But she's just a character in a play.
Heyoke stares back in horror, he had no idea things had gotten so bad since she locked herself in the library.
Heyoke (horror on his face): How long are you going to keep doing this?
Serinth: As long as it takes.
Heyoke (nearly whispering) You have to stop.
Serinth (wry smile): I can't stop now. This could be my debut.