Mizuhashi was told during auditions to try out as many characters as she wanted. She picked Mami because the name had a sort of magical girl image to it. She thought that she hadn't done many onee-san type characters, so it would be fun and she wanted to switch her voice acting roles. She also auditioned for Kyousuke Kamijou and Kyubey, but not for any of the other main girls, as she thought she didn't portray their image.
She knew that Mami was set to die after three episodes, but not how it would happen. When she read the scenario she wondered what she should do for the scene of Mami's demise, since she hadn't voiced too many characters that die. She thought that the way Mami dies is portrayed in a relatively real manner, so she decided to keep acting normal until the last moment. She was satisfied with this type of death as it made Mami a memorable character.
Kaori's page from the Rakugaki Note Rebellion
Kaori's page from the Tiro Finale Book
Kaori's art from Puella Magi Madoka Magica Pia