Ahaha this may be too much of a meta-dump for a cameo BUT
I think the thing to realize about Éponine is that she's not crazy. She's a bit disjointed, sure, and there are times when even she doesn't realize her own motivations, but that doesn't mean she doesn't have them. book!Éponine... what some people don't pick up on is that she's perfectly aware of her own physical, moral, and social degradation, and post-Marius she's intensely ashamed of it. It's a matter-of-fact kind of shame, but it's still there: it's what drives her to stop speaking argot, it's what makes her so eager to prove to Marius that she can read and write, that she wasn't always meant to be what she is now. It's what's behind her offer to walk at a distance from him, so he won't have to be seen with "a woman like her." Seeing Marius, who hasn't been corrupted by his poverty, gives her the will to reject the whole life her parents have dragged her into even as it makes her extra-conscious of how much she's been corrupted. All of which leads into her confrontation with Thénardier and Patron-Minette at the garden gate, where she openly defies him--and where her lack of self-worth is what makes her fearless, because she really doesn't care if they kill her. (And again, when Mabeuf calls her an angel for watering his plants--her response is "no, I'm the devil, but it doesn't really matter.")
So ultimately she is almost a Valjean character: she's deliberately shifted her alignment from chaotic evil to chaotic good, but without gaining any sense of redemption or conception of herself as a good person. In my head I suppose she thinks of herself as a devil working on the side of angels as best she can, and she's not afraid to use the means and methods from her criminal upbringing towards her own ends, since unlike the people she's appointed herself to protect, she can't be tarnished any further by that association. And, of course, she's not above selfish actions, but I think she conceives of the whole "die with Marius on the barricades" plan as a romantic and not a criminal gesture, and doesn't realize how fucked-up and wrong it is until she's in the middle of it.
UMMMM not that any of this will help with writing her dialogue and mannerisms. But it might help with her motivations and how she would realistically react to things? And I mean, when I say shame, she's definitely not a shrinking violet about it--she's been around the block a few times and she's totally brazen, but she knows how screwed-up that is and what a sad and ghoulish figure she cuts to an observer. And it might help pin down her dialogue to go back and read some of her scenes in the novel from that perspective.
[Dieu] entend ta voix, ô fille des hommes! aussi bien que celle des constellations; car rien n'est petit pour celui devant lequel rien n'est grand.
- George Sand, Les sept cordes de la lyre