I remember reading an article on Anime News Network awhile back; when they were auditioning for the part of Suigintou, Tanaka Rie showed up wearing all black; though her voice type wasn't exactly what they originally had in mind for the part, they chose her because her mannerism fit the personality of the role.
Speaking of the voice cast, back before the anime, there was a drama CD made that was based on the manga, the cast listing included:
Shinku - Yui Horie.
Jun - Sanae Kobayashi.
Nori - Aya Hisakawa.
Suigintou - Mamiko Noto.
Hinaichigo - Tomoko Kaneda.
Tomoe - Ayako Kawasumi.
Suiseiseki - Nana Mizuki.
Laplace's Demon - Joji Nakata.
The list might sound a bit weird, but considering that it was made based on the manga, before the anime aired, it actually sounded rather good.
Here is a clip of Sawashiro Miyuki at the 2011 Anime Expo:
http://www.youtube.com/watch?v=oBhlM0UpHLU