While Tilda Swinton does another one of her typically marvelous performances, it is Amber Tamblyn who displays the surprisingly great acting chops in this film. I must admit that although I knew of her for her TV role in "Joan of Arcadia" and for the movie, "The Sisterhood of the Traveling Pants," let's just say that I don't think a 46 year old guy is typically in her target audience. After this role, however, I have no doubt that if she keeps her head on straight, she's the next great actress of her generation.
As other commenter's have noted, her scene in the public bathroom on the ski trip is at times uncomfortable because of how devastatingly powerful it is. Tamblyn's performance deserves a Supporting Actress Oscar nomination at the very least, but because this film has barely been seen (even seemingly by the frequent movie-going audience of IMDb!), I doubt it's even on most people's radar.