Susan on Glinda and Elphaba (from SusanHilftery.com)

From SusanHilferty.com (check it out for info on Wicked's Costume Designer, Susan's sketches, interviews, and more!):

Glinda is the epitome of good, so I did research by asking little girls what goodness looks like. They said like a princess, like a bride. I collected and studied pictures of Queen Elizabeth II from her coronation, Lady Diana’s wedding dress, and all of the dresses that are emblematic of perfect femininity. When you look at any of the English coronation images, it’s hysterical, because it’s all about impressing in a certain way. Even Queen Elizabeth, in the 1950s, wore a crown and a long robe and held her scepter, and I wanted to tap into that. Glinda is also connected to the sky, sun, and stars. That influenced her tiara and wand. The sparkles on her dress are all about that, too. She symbolizes lightness, air, bubbles. Kristin is someone you know loves costumes, because she comes from a beauty pageant background. She can wear any kind of heel, and she knows makeup and hair. But she wanted to be transformed to be Glinda. We did a lot of fittings. I had to find just the right proportion in her Glinda dress, because she is tiny, tiny, tiny. She can get swallowed up in a second. It’s like one ruffle too many and you’re over the top.

I didn't even know that about Kristin being in pageants... you'd think she's too short; I thought they had to be like eight million miles high and two wide. Haha.

Elphaba is exactly the opposite. I see her as connected to things that are inside the earth. So the patterns and textures I wove into her dress include fossils, stalactites, or striations that you see when you crack a stone apart. I mixed different colors into her skirt, so everything is literally twisted. And there was no issue with Idina about whether a costume was or wasn’t flattering. Idina is unbelievably gorgeous, but she’s not at all obsessed with her looks. When I designed her clothes for Shiz, I gave her heavy boots, so right away she’s connected to the earth, and then a cap that she pulls down low. Plus, she keeps her hands in her pockets. Idina completely embraced the idea of a young girl trying to hide herself. Now, by the time she gets to the Emerald City, she feels she belongs. I change her shoes so that she has a lighter pair. We take her glasses away, her hair comes down, and she’s wearing a lighter color. And suddenly she feels accepted and even, you could say, fashionable. Glinda tells her at the end of Act I not to be afraid and she answers, “I’m not.”