Gene and Phineas are the two characters we will want to look at in analyzing the ideas of guilt and grief in the novel. The first major event in the novel generates guilt for Gene and grief for Finny - when Gene causes Finny to fall from the tree, breaking his leg.
Gene does not initially know what made him shake the branch and knock Finny from the tree. Later, he comes to the conclusion that simple jealousy led him to do it and he is challenged to deal with the guilty idea that the act was not an accident.
Gene's impulse, at first, is to stay quiet about what happened and what he has done. Soon, however, he is driven to confess. His guilt is torturing him, so he goes to Finny's house and tells the truth about knocking Finny from the tree.
My own voice sounded quiet and foreign "I jounced the limb. I caused it." One more sentence. "I deliberately jounced the limb so you would fall off."
Finny does not have the same impulse toward honestly facing the notion that his best friend knocked him out of a tree.
Finny reacts with disbelief and becomes very agitated, unable to accept the enormity of the idea that his friend harbors such a virulent hatred against him...
Finny escapes into denial in his grief over Gene's act of betrayal, as well as the loss he suffers regarding his athletic future, his ability to enlist and his identity. (Having been largely defined by his physical ability and grace of movement, Finny's identity is essentially taken from him when he breaks his leg.)
Part of Gene's self-imposed penance includes opting out of sports. Gene attempts to become the crew manager, but after a fight with the head manager and a conversation with Finny, Gene decides to train for the olympics for Finny, again as part of the penance for his guilt.
In this way, Gene attempts to deal with his guilt.
For his part, instead of honestly dealing with his grief, Finny prefers to deny Gene's betrayal just as he publicly denies the existence of the war and chooses to doubt the real existence of the Latin.
Finny does acknowledge the reality of the war, eventually, demonstrating that he is aware of more than he lets on. Yet he prefers to suppress any acknowledgment of Gene's role in the accident that broke his leg.
The final act of this suppression precedes Finny's fall down the stairs after he leaves the trial Brinker has arranged. Finny had been fleeing the truth in that moment as he rushed to the stairs.