they both were in a fake dating with each other. They were enemies

He needed to date someone to get his dead fathers money and she needed to date someone to prove to her old and almost mother that she could be with someone, so she would inherit the fortune. So they, even "hating" each other, decided to do get what they wanted

They were in the Slytherin Communal, where a big party was happening. After more than half of the people went away, their friends started playing truth or dare