the heart wrenching scene where Katniss takes her sister's place at the reaping is one that first reveals that her passion and duty are fully wrapped in protecting her family.  She clearly does not even take a moment to think of herself, but, almost insanely volunteers to protect her sister.

It is easy to look at Katniss, the victor of the Hunger Games, and assume she is strong, fearless, and cunning.  But both the book and the movie do a great job of showing her trepidation in the games, her uncertainty, and relief every time she succeeds.  It is obvious that much of her success is due to luck (or fate) and an inner passion to simply survive.  The film does not reveal any sense of a killer instinct, but rather that her compassion and youth is only outweighed by a deeper sense of making it out alive to, once again, go home and provide for her mother and sister.  Even her skills with a bow and arrow do not give her a sense that she is stronger and better than the others.  She is clearly very afraid.  

However, the film also shows that she does not reciprocate love for Peeta.  It is clear she is both uncomfortable in pretending to love him, and worried that someone can tell she's lying.  In scenes with Peeta and Katniss one-on-one, Katniss remains very focused on the goal to simply stay alive, whatever it takes.