I haven't seen the movie and I think a true visual usually helps you get a good grasp of attitude.
Finny sat quiet in chapter 11 during the interrogation or trial. I think he was pensive... thinking. Brinker had just in the last chapter and a half put Finny in his place a couple of times. This might have knocked Finny down a little bit destroying whatever shreds of confidence or dignity he had left after having his leg destroyed and his dreams of intense athletics or participation in the war shattered. When the trial went the way it did, Finny had nothing left and he lost it. Most of all he felt betrayed.
In A Separate Peace, chapter 11, Finny experiences drastic changes of mood and attitude during the trial scene.
At first, he is perplexed:
"You see how Phineas limps," said Brinker loudly as we walked in. It was too coarse and too loud; I wanted to hit him for shocking me like that. Phineas looked perplexed.
When called to testify, Finny shrugs, a gesture suggesting that he'll go along, say, just because. He is noncommittal at this point. Finny is flippant and a bit sarcastic when Brinker tries to question him, however. He is resistant.
His resistance changes, though, when Brinker asks Finny if he's ever thought that he didn't just "fall" out of the tree. Gene, as narrator, writes:
This touched an interesting point Phineas had been turning over in his mind for a long time. I could tell that because the obstinate, competitive look left his face as his mind became engaged for the first time.
When the spotlight turns on Gene, Finny tries to defend him, suggesting that Gene was at the bottom of the tree. But when the subject of Leper comes up, it is Finny that reveals Leper is actually on campus.
After Leper testifies and Finny realizes and apparently admits to himself that Gene is responsible for his accident, he is hurt and probably broken hearted. He gets up silently and moves to leave. When Brinker tries to stop him,
The words shocked Phineas into awareness. He whirled as though being attacked from behind. "You get the rest of the facts, Brinker!" he cried. "You get all your facts!" I had never seen Finny crying. "You collect every f--ing fact there is in the world!" He plunged out the doors.
Of all of Finny's illusions destroyed in the novel, the illusion of Gene as friend, and the destruction of that illusion, is the one that makes him cry.
In the book "A Separate Peace" Gee who was jealous of his friend Finny jumped a tree limb while Finny was on it and it resulted in Finny falling ad breaking his leg so badly that he could never walk the same. Gene had tried to tell Finny the truth initially, but Finny was not ready to hear it.
Brinker calls a mock trial and brings in Leper as a witness because he wants Finny to know the truth. Finny loves Gene and initially does not want to hear it. However, once the truth has been told he becomes upset and leaves the trial. The next thing that occurs is he falls down the stairs re-injuring him. The is some level of inference that makes the reader wonder if he had tried to kill himself or had become so upset that he had just fallen.