Mary Crawford is really an opportunist chameleon, sucking up to the right people at the right time in this chapter.

When she knows that Edmund is against them doing the play, but on the other hand she wants to but has no "Anhalt" as a partner in the play, she goes up to Edmund:

“If any part could tempt you to act, I suppose it would be Anhalt,” observed the lady archly, after a short pause; “for he is a clergyman, you know.”

And then, there's poor Fanny later in tears because everyone is trying to force her to play "cottager's wife":

"Edmund was too angry to speak; but Miss Crawford, looking for a moment with astonished eyes at Mrs. Norris, and then at Fanny, whose tears were beginning to shew themselves, immediately said, with some keenness, “I do not like my situation: this place is too hot for me,” and moved away her chair to the opposite side of the table, close to Fanny, saying to her, in a kind, low whisper, as she placed herself, “Never mind, my dear Miss Price, this is a cross evening: everybody is cross and teasing, but do not let us mind them”; and with pointed attention continued to talk to her and endeavour to raise her spirits, in spite of being out of spirits herself. By a look at her brother she prevented any farther entreaty from the theatrical board, and the really good feelings by which she was almost purely governed were rapidly restoring her to all the little she had lost in Edmund’s favour."

Later, when they decide to get an outsider to play "Anhalt" because Edmund refuses to, Mary says to Fanny:

“I am not very sanguine as to our play,” said Miss Crawford, in an undervoice to Fanny, after some consideration; “and I can tell Mr. Maddox that I shall shorten some of his speeches, and a great many of my own, before we rehearse together. It will be very disagreeable, and by no means what I expected.”

Is it not possible for Mary to really take a stand on anything? I think the best word for her is opportunistic. She knows how to woo the right people at the right time for her purposes.