This isn't the first time Bella's life has depended on the decisions of a vampire. It isn't even the first time Edward has protected her from a crimson-eyed (she tries, in vain, to steer her mind from the word evil) vampire.
But this might be the first time the outcome will depend on something other than Edward's strength and battle tactics compared to his foe's. On his ability to persuade, to find loopholes in draconian vampire law. He seems to have a shot - he reminds Caius of Gianna, the Volturi's all-too-human receptionist.
Caius smiles. Bella doesn't like that.
"When they are no longer useful to us," he tells Edward, in the tone of a teacher trying to embarrass a student, "they will serve to sustain us. Is that your plan for this one? To destroy her, if she betrays our secrets?"