I was completely won over by Jennifer Lawrence's portrayal of Tiffany in this movie. I don't know how much of her character's charm was present in the book or screenplay, but I believe the actress did a phenomenal job bringing Tiffany to life.
I am happy to know that Lawrence has been recognized for the part, winning both the Golden Globe and SAG awards for best actress, and I find myself rooting for her in the upcoming Academy Awards. Some details from her performance that I particularly enjoyed include... (SPOILERS!)
...Tiffany's mercurial emotions and her lack of shame about that.
...The heart-wrenching ugly cry she gives when her sister shows up at the competition with Nikki.
...The way she didn't take crap from other characters. She didn't even blink before throwing sass back.
I was also really happy with the handling of Nikki. She wasn't made a villain just for rejecting the protagonist or for cheating on him. She was given a dignified, quiet conclusion rather than some kind of punishment/humiliation, which is the story I'm used to seeing in movies.