She's pretty sure she and William aren't meant to have the sort of relationship where you just show up at each other's houses, but they do. (It's not like-- that. Like the rumours imply. They're just really good friends.)
It's an ordinary morning, when she knocks at his door. She's managed to get ahead on some of her classwork-- which is a bit of a feat, for a medical student, but getting ahead and having time to spend with people is important-- and she feels like seeing how his latest composition is going.
So she knocks, and expects him to answer.