I'd like to know the same thing about Sarah Rice's interpretation. This is actually a big problem I have with the show, and it apparently started at the conceptual level. The writers can't seem to decided whether Johanna should be taken seriously as a portrait of threatened innocence, or played for parody--their view of her changes every other scene. "Green Finch" is a lovely song if played seriously, but it becomes actively annoying when overdone (by Betsy Joslyn, IMO), while "Kiss Me" requires breathy ditziness. I think this is because Victorian notions of virtue and purity seem sappy to us now, and it is difficult to know just how to play them believably for modern audiences.
Personally, I prefer a more "real" approach. Johanna is the first person we see in the show who isn't corrupt or twisted by life in some way. She is also the person everyone is fighting for; everyone has a stake in her fate (directly or indirectly) and her name is mentioned in the score almost as many times as Sweeney's is. If she and Antony are just normal, ordinary people in trouble, they provide a foil for all the other big, outsized personalities to bounce off of.
I ask in all honesty/What would life be?/Without a song and a dance, what are we?/So I say "Thank you for the music/For giving it to me."