Phaedra was not going to interfere, at all, until she saw the knife. She did not care if waking this person might be dangerous, or get her stabbed.
She could survive that. A human behind the door--and she fished with her mind to find out who's door it was, just a quick reach out to see who was behind it--possibly would not.
In the space of an eyeblink, Phaedra was behind the man with the knife. She had made no sound, and stood still as a statue there behind him.
Then she put her hand on his right wrist and held it. Her grip, she'd been told, felt like a cold vice.
She would start slowly. This might not wake him, but it would possibly stop him on this errand. Phaedra wasn't hungry, but her eyes did note the pulse in the sleepwalker's neck.