In the story "A&P," I feel that Sammy is a round character because he changes once he has his eye on a girl. I think Queenie is the flat character in the story. We do not get to know much about her or what she is thinking. I'm just trying to determine if I'm on the right path with these two as to being round or flat characters.
In "A & P" by John Updike, Sammy is the protagonist. The story is told from Sammy's point of view, and we find out much more about Sammy than we do about the other characters. Thus you are absolutely correct in saying that Sammy is a rounded character and the other characters are flat. This effect is strengthened by the nature of Sammy as a narrator. He is a self-centered adolescent, who has little respect or understanding for the customers at the store, as we see in the beginning of the story when he is serving an older woman who carefully watches him ringing up her purchases. Rather than understand that she may well be someone for whom every penny counts and sympathizing with her anxieties, he is rude and contemptuous to her because she is not young and pretty and scantily dressed, unlike the girls on whom he is focused.
This flattening effect also occurs as our Sammy observes Queenie and her friends, mainly through the lens of his emerging sexuality, in an example of what literary critics sometimes call the "gaze". Although as readers we can look beyond Sammy's preoccupations and see that the other characters in the story are people in their own right, Sammy as a narrator only sees them as they impinge on his own needs and desires, thus making them into flat characters in the story.