From Inside the Grey House:
“What decides the attitude of a person when they are turned?” Her words sounded serious. This was apparently something she thought of often.
Vincent looked down into her eyes, finally understanding part of the hesitation he had always sensed in her. His hand went to her hair, tenderly caressing the nape of her neck. “A human’s darkest desires determine how devilish she is when she is turned. Her strong humanity acts as a counterbalance to even it out.” He paused as the words sank in. “In other words, nothing changes unless the human wants it to change.”