People often talk about a character arc plotting the changes of a character because of what happens to them. I prefer to think of plot as not changing the character but revealing it.
We see Sam’s character when he is presented with the opportunity to join the pressers. He refuses because he knows his father, mother and siblings need him to tend the farm. Imagine a teenager presented with the offer of freedom, independence, and more importantly, the chance of getting a girl. Sam has to turn away from all of that to face a life under the thumb of his father, working the fields, and without any hope of meeting a girl. That is character. From day one he thinks of others and does what he thinks is right.
Of course, he changes: he loses his innocence, he sees battle, he sees death, and he is not a superhero. But these changes are really just the light shining on a different facet of his humanity. We need to see he is a human, not Captain Marvel. Death scares him. The idea that a horde of Northman want to kill him, makes him so scared he can barely get out of the privy. I like that glimpse of his humanity. But what is important is that he rises to the challenge. He does his duty. Scared to his bones but willing to fight as best he can.
Everything that happens to him reveals another facet of his character. He was always a hero, even when he was just bent over his father’s knee getting a spanking. Nothing changed but the light shone on him. To see a character we need the light of circumstance to shift so we see hidden facets glimmering. We see Sam when is powerless and when he is powerful. We see him tempted, hurt, experiencing sorrow and joy.
He seems incorruptible. There is a moment when he has done well, earned respect, he has status, but then he trips over his tongue when confronted with a beautiful girl, and he falls tearfully into his mother’s arms without any thought to how people will view his manliness for it. Of course, all humans are corruptible… but that’s more of a book 2 thing.