Sarty struggled with his morality as it was in direct conflict with his desired loyalty to his father until, eventually, his morality won. He is round and
dynamic. As he was called as a witness, Sarty felt obligated to recognize his fathers "enemies" and to lie about the barn burning (paragraph 1). Along the trail, Abner slapped and threatened Sarty, that he must "stick to own blood"(paragraph 28), meaning they were all family and they had to stick together. Sarty knew his father's acts were unacceptable but also wanted to be loyal to his father. Later, when Major de-Spain's demand for repayment for the ruined rug, Sarty strives to be a true-to-form Snopes as he proclaims de-Spain will not get even one bushel of corn (paragraphs 64, 79). All through Sarty's character development, he changes ever so slightly until his morality wins over. His attempt to warn de-Spain is in vain. Without really knowing the outcome, he turns his back on a life style he could not live, thus changing his character type to round and dynamic.
As Sarty is moral in nature, his father is not. His father never changes and is flat and static. Not only is Abner an abuser, but an arsonist as well, using fire to gain his revenge. Throughout their life, the family is constantly moving, Linnie always crying for Abner to not do it again, and it always seemed to Sarty his father always made new home arrangements before they had to leave. . However, his attitude is necessary for the plot. Their arrival at the de-Spain's residence gives Sarty some hope. Sarty was sure this example of a lifestyle would change even his father, that this place would be "impervious" to Abner's burning. It didn't. Eventually Abner and his oldest son leave for the de-Spain's barn, with containers of fuel. Abner is a flat and static character because his attributes never change Major de-Spain is depicted as a dominated man and intimidated by his wife. As expected, he also does not change. Major de-Spain is also flat and static. Major de-Spain explains his wife would not be quiet about the incident but maybe by charging Abner twenty bushels of corn, Abner would learn to "wipe his feet". Major de-Spain is henpecked, an obvious flat and static individual.
So, Sarty is rounded and dynamic because he develops and changes his ideas of morality. His father remains the arsonist and continues to sidestep responsibility for his actions. He never changes, thus is flat and static. Major de-Spain is not really a major character (flat and static participants usually aren't) but he is the next most active participant. As he is a dominated person (by his wife) and will not change, he is flat and static.