Turns out she's paying more attention to the Slytherin social happenings than the Gryffindor ones. Gryffindor is also having a party Friday night, and Hermione takes an opportunistic moment to snatch a 1/4 bottle of firewhisky off Lavender's side table while she and Parvati get ready. The burning liquor does bolster her nerves and her courage, and she takes another strong swig before making her way back to the library.