Looking back, it probably began the day she was sorted into Ravenclaw instead of Gryffindor.
On the surface, she'd been thinking Gryffindor, like her father had wanted, but deep down, she'd known Ravenclaw would suit her better. Rose had read everything she could about Hogwarts, ever since her mother had taught her how to read, and despite all the family stories, she'd known she was better suited to the bird than the lion. It had helped that, a few minutes before, she'd watched Al being sorted into Slytherin, and Scorpius Malfoy being sorted into Ravenclaw. She knew she wouldn't be alone in disappointing her parents.
In the common room, after the feast, she'd sat down at one of the desks to start composing a letter. He'd sat down next to her then, looking stern at first, and then returning her shy smile.
Come to think of it, maybe it had started even earlier, on the train.