What was it like playing a crazy character like Agnese?
[POSSIBLE SPOILERS] I loved playing her. Though she be but little, she is fierce. As a voice actor, however, I only get glimpses of her for seconds at a time. The whole story never becomes realized until I see a finished product. (I don't always watch an anime in Japanese and then get cast in a role on the show. I feel that it lends itself toward creative plagiarism to try to copy someone else's performance. Not cool.) I can answer it retroactively, but to do so, I have to argue against the question.
A crazy person never thinks she's crazy. Agnese believes that she is doing the right thing; her religious zealotry is what makes her an effective leader and gets the other nuns of the Church of the Cross follow her almost blindly. I never her approached her as insane, just very committed to her convictions. She lies to Toma and Index about her true motives, but she sees it a necessary way to recover the Book. In flashbacks, we know she was out on the street on the Church took her in and cared for her. Perhaps in exchange, she pledged herself wholeheartedly to the cause, no matter what it may actually fight for or knowing fully what it would ask of her. One fights to protect family, and this just happens to be hers.