During the seven days that Sam is forced to repeat, until she is able to change what she wants to change, we see a gradual but definite development in her character. After the first couple of days, we see that she adopts the kind of attitude that tries to convince us she doesn't care about the problems she is forced to witness and be part of every day again and again. Note how she expresses this selfish attitude:
It feels like I’ve been caught up in some enormous web and every way I turn I see that I’m stuck to someone else, all of us wriggling around in the same net. And I don’t want to know any of it. It’s not my problem. I don’t care.
However, as the day continues to repeat itself, and after hearing about Juliet's suicide and then seeing her with her family one night when Sam chooses to go to a restaurant with her family instead of going out with her friends that Sam resolves to change her selfish attitude and to try and use this repeating day to do something good and to save Juliet:
I realize I’ve never really done something good for someone else, at least not for a while.... This will be my good thing.
From this point onwards, we see a marked change. Note what Sam says about herself after, instead of joining in and mocking people that she and Lindsay bullied before, she erases graffiti about them:
It feels like I’ve reached back in time and corrected something. I haven’t felt so alive, so capable of doing things, in I don’t know how long.
This change in Sam's character is of course most definitely seen when she gives up her own life by pushing Juliet out of the way of the car and dying in her place. Sam has finally had the courage to be herself rather than thinking of how others view her.