I feel Mary gets pleasure out of manipulating people. She manipulated Fanny first into accepting the gift of a gold chain and then into choosing one Henry purchased. She calls Fanny to “the duty of remembering the original giver [Henry]” (26) and then laughs at her embarrassment and pretty conscious and ridicules her scruples until Fanny ‘dared not make any farther opposition’ (26) to prove she suspects no trick. I think it was the joy of the game, actually succeeding at the game that Fanny saw in Mary’s eyes. In Ch. 18 Mary is very pleased to tell Fanny she talked Mr. Rushworth out of his jealousy for a few moments:

By the bye, I looked in upon them five minutes ago, and it happened to be exactly at one of the times when they were trying not to embrace, and Mr. Rushworth was with me. I thought he began to look a little queer, so I turned it off as well as I could, by whispering to him, ‘We shall have an excellent Agatha; there is something so maternal in her manner, so completely maternal in her voice and countenance.’ Was not that well done of me? He brightened up directly.” (18)

I suspect she probably did this as much for herself as Henry or just the pleasure of doing it because Mr. Rushworth’s jealousy could have ended the play and neither Crawford was ready for it to end. In Ch. 16 Mary first tried to flatter Edmund into acting and then appealed to his gallantry which finally works—I think she was manipulating him and there were not ‘not fewer smiles at the Parsonage than at the Park on this change in Edmund’ (17). In Ch. 4 Mary describes how she, her aunt and her three friends and their mothers all tried to trick Henry into marrying one of the friends—it seems even her brother can be the object of manipulation.