We are limited for space, so I'm going to focus on just one or two characters. However, first lets establish that all the characters are able to travel through all 5 stages of grief by the end of the story.
The 5 stages are: Denial, Anger, Bargaining, Depression, Acceptance
As is the case in real life, the characters of the story don't move from one to the other in a clear line. There is always going to be some back and forth movement. Even after acceptance, a person can travel back into depression.
Of the living characters, Jack and Lindsay experience the stages in the most developed way. Jack refuses at first to accept that Susie might be dead. When he is presented with the body part and must accept, he does so for the sake of his family. But he quickly retreats to anger. He smashes all the ships that he and Susie worked together to build. He shifts into bargaining as he finds a target for his anger in Mr. Harvey, and works with the police - harrasses the police - to get Mr. Harvey arrested. He bargains with Lindsay to find more evidence. His anger comes out again in the cornfield when he attacks who he thinks is Harvey. After ending up in the hospital, his anger slowly subsides into depression, a depression so deep that Buckley, left alone at home with his dad, feels alienated and ignored. Finally, at his heart attack, Jack starts to let Susie go and accepts a life without her.