Scene that always stuck with me (SPOILERS)

Towards the end, when Bernard starts yelling at Joan that he'll sue her, he then tries to force the boys into his car so he can take them back to his place. Frank then refuses to go with his father, and Bernard tries to physically pull him but Walt jumps in between them and tells their father that Frank doesn't want to go, and he'll go with him.

That scene stayed with me cause its such a turning point for Walt's character. Everyone else in the family essentially has remained the same by the end of the movie: Bernard still a narcissistic jerk, Joan still a sweet mother but also a cheater, and Frank was unfortunately an emotional wreck to begin with. Walt is the only one we really see make a change, and it really kicks into gear here when he defends his brother (his oft-opponent) and manages to get his father away from his mother and brother, whom at this point are probably a little scared of him. He goes back to being his lackey, naturally, when he witnesses his father have a heart attack in the middle of the street. But then quickly realizes his selfish ways after his father uses his heart attack, just like everything else in his life, to his advantage to gain sympathy and camaraderie from his son.