She never shows fear around him.
He was trying to play her. She was trying to play him. She never answers a direct question. She never does what he commands and plays it off as a conversation.
She does not fear questioning his approach to twisting the wizard, pointing out how fear is a brittle weapon if that's the only thing he ever deigned to use. She does not fear his claws at her abdomen. He is a sadist, a torturer. He is testing her to see if she'll break. She doesn't think he'll kill her, at least not right now.
Then she sees the cleric pull the sunsword from the ground and attempt to drive it into his back. Azrael is defiant, trying to show she does not fear him, but in that moment, it becomes a lie.
There is one thing she fears from him, and it has nothing to do with what he might do to her own person. She tries to shake her head no, to signal to the cleric that she has this in hand. Azrael tries to appear more blase and confident than she feels right now because she does not want to admit to either of them standing before her:
There is one thing she fears. One weakness that Strahd must never be allowed to use against her because it would work, and that weakness was attempting to literally stab him in the back.