I thought Cathy was supposed to be very beautiful?
No offense, but Kiernan doesn't cut it. At all.
"Hey guys! Whoa, Big Gulps huh? All right! Well, see ya later!" Dumb & Dumber
Okay, really, insulting a veteran teenaged actress because she doesn't fit your ideal of being pretty or your vision of what the character in the book may have looked like? I think she did a fine job and probably got the part because she's played in a emotionally complex role in "Mad Men." Maybe a different actress might have been considered more attractive, but just may not have been able to pull off such a conflicted character. The one I thought had difficulty with his role was Mason Dye, not necessarily because he was a bad actor, but he seemed really uncomfortable with the material- and it showed. I haven't seen him in anything else so it's likely he just wasn't seasoned enough to pull off such a character.
Plus, he was directed to act like a Stepford child who seemed to act like, "Oh, it's normal to be locked up in an attic by my loving mother and mean grandmother and stay here till our grandfather dies." It seemed like Kiernan portrayed well that she wasn't drinking the Koolaid (or eating the arsenic) that her brother seemed to have ingested. Thankfully, he finally got it and had a long overdue rage at the mother and grandmother and acted in the rest of the movie pretty well until the character of Cathy begged him to leave and he still felt conflicted.
I honestly didn't mind Mason Dye either. How you described him acting is exactly what Christopher was supposed to be in the book. Hell in the second book he still is conflicted about their mother - it takes until the very end of the book (I won't spoil what happens in case you haven't read it) for it to get through to him and by this time he's in his 30's.
"It's better to be hated for who you are than be loved for who you aren't."