Final answer:
In 'A & P' by John Updike, Sammy is a round character who undergoes significant changes, while the customers and Lengel are flat characters who remain consistent throughout the story.
Step-by-step explanation:
In John Updike's short story A & P, the characters vary in their complexity and development. Sammy is a round (dynamic) character because he undergoes significant change, particularly in the way he starts questioning societal norms and eventually quits his job. This shift in his perspective on life and personal rebellion against conformity demonstrates his transformation from beginning to end.
The customers and Lengel, on the other hand, are flat (static) characters. The customers do not undergo any significant change; they continue shopping as usual, largely unaware of the dramatics unfolding around them. Similarly, Lengel remains the strict, by-the-books store manager throughout the story, serving as a contrast to Sammy's evolving mindset.