A slight bit of Mood Whiplash, as Glinda gets in a laugh before getting serious;
Elphaba: You're the only friend I ever had.
Glinda: I've had so many friends...but only one that mattered.
Page 78 in the book, where Galinda stares at Elphaba in their dorm, who in turn is staring out the window:
They both gazed at the green and flowery spectre reflected in the watery old glass, surrounded by the blackness, driven through with the wild rain beyond.
A maplefruit leaf, shaped like a star with blunted points, or like a heart grown lopsided, suddenly whirled out of the night and plastered itself on the
reflection in the glass, gleaming red and reflecting the firelight, just where the heart would be - or so it seemed from the angle at which Glinda stood.
"Entrancing," she said. "There's some strange exotic quality of beauty about you. I never thought."
Two moments during "One Short Day in the Emerald City";
Just before the "Wiz-o-Mania" performance, Elphaba gushes that she always wants to remember what it's like being in the Emerald City where no one points at her green skin as being odd. Glinda smiles and says, "Elphie, you look positively...emerald!"
At the end of the song, when Glinda says they're just "two friends" touring the Emerald City, Elphaba hesitantly adds, "Two good friends". Glinda gently corrects her; "Two best friends".
Before "As Long as You're Mine" in the musical:
Elphaba: I wish I could be more beautiful. For you. And don't say that I am, you don't have to lie to me.
Fiyero: It's not lying. It's seeing things differently.
And the reverse of that. Elphaba's "you're still beautiful."
Watching the last third of the second act in the Musical, knowing the ending. Knowing what the note Elphaba gets REALLY reads. And what is REALLY driving her subsequent actions. It isn't despair. It is hope that she had long since cast aside.
A subtle one but think about it: Nessarose's only reaction to Boq's transformation is mild shock at first and then pinning all the blame on Elphaba. How is this heartwarming? Because she could have very easily rejected him. But she doesn't. That's right, Nessa still loves Boq, still wants him to be with her and doesn't think any less of him, even after he's been turned into a subhuman being...and rejected her. That's real love and dedication right there.
"Dancing Through Life". Boq and Nessa. That is all.
Boq: Let's dance.
Boq: Let's dance!
And then when Glinda finally gets the nerve up to dance with Elphaba.
When Elphaba does her poor approximation of dancing and Fiyero remarks that she certainly doesn't care what anyone thinks.
Galinda: (holding up her new wand) Of course she does. She just pretends not to.
In the fourth book, Rain, who didn't pay much attention to other people as a child, has her earliest memories of playing with some animals by herself. At the end of the book, she confronts Glinda with her lonely upbringing - only for Glinda to remind her that she wasn't alone - Glinda was with her every time. She fills in the gaps in Rain's memories, and in doing so completely changes the nature of their relationship.