Sylvestra Le Touzel may not have been perfectly cast, but she was a better Fanny than Frances O'Connor in Mansfield Park (1999) or Billie Piper in Mansfield Park (2007). I don't know whether this was the fault of the actress, the director, or the writer. I highly suspect that Patricia Rozema wanted Fanny to be like Jane Austen, not the shy and timid Fanny in the novel. It may have been the writer Maggie Wadey who was responsible for the childish Fanny in the 2007 adaptation. I suspect that the director told Sylvestra how to act in the scene where she would not accept Henry Crawford's proposal.

]But she also constantly appears frumpy, silly, slow of understanding, almost as dazed as Lady Bertram and generally not very likeable. ...... The scene with Mr. Betram pressing Fanny to accept Mr. Crawford was embarassing because of Fanny's explosion of feeling which was so awkwardly performed. The level of screaming alone had me turn of the sound - badly done!

Well, perhaps she is a bit frumpy, but not "silly, slow of understanding, almost as dazed as Lady Bertram ..." Lady Bertram was very dazed, silly and slow of understanding. Fanny had to keep some secrets regarding 1) the stage play kiss between Henry Crawford and Maria Bertram and 2) her love for Edmund because he was in love with Mary Crawford. It turns out that Fanny understood the Bertrams better than anyone in the Bertram family. In Chapter 32, Fanny did have an "explosion of feeling": There was a look, a start, an exclamation on hearing this, which astonished Sir Thomas; but what was his increase of astonishment on hearing her exclaim—”Oh! no, sir, I cannot, indeed I cannot go down to him. ..." Sir Thomas came towards the table where she sat in trembling wretchedness, and with a good deal of cold sternness, said, “It is of no use, I perceive, to talk to you. We had better put an end to this most mortifying conference. ..." Fanny was by this time crying so bitterly that, angry as he was, he would not press that article farther. Her heart was almost broke by such a picture of what she appeared to him; by such accusations, so heavy, so multiplied, so rising in dreadful gradation! Self–willed, obstinate, selfish, and ungrateful. He thought her all this. She had deceived his expectations; she had lost his good opinion. What was to become of her?