The Encyclopedia Britannica describes round characters as follows:
Flat characters are two-dimensional in that they are relatively uncomplicated and do not change throughout the course of a work. By contrast, round characters are complex and undergo development, sometimes sufficiently to surprise the reader.
In Susan Glaspell's one-act play, "Trifles," Mrs. Peters is definitely a round character.
At first, Mrs. Peters being the sheriff's wife, supports him in his job of upholding the law. She also does or says whatever society dictates she should do or say to be a respectable female in a male-dominated society.
Several times in the story, the men make careless, thoughtless comments that are meant to diminish women. For example, when the women express Mrs. Wright's concern that her preserves might have frozen, Mr. Hale refers to women's chores as "trifles." The stage direction indicates that the women, Mrs. Peters and Mrs. Hale, move more closely together, showing that this moment begins their sympathy for Mrs. Wright and their solidarity to do what they can to protect her from the world of short-sighted, uncaring men.
At another point in the play, the men make fun of the work women do as if it is nothing, and particularly laugh over the job of making a quilt.
Mrs. Peters is uncomfortable with the comments made, but she is a woman used to doing as society—and her husband—expect her to. She is, at first, hesitant to change her personal beliefs regarding a woman's responsibilities, and the law. She tries to convince herself that Mrs. Wright is at fault, without question.
However, when Mrs. Peters and Mrs. Hale discover the dead canary, with an obviously broken neck, they start to believe that Mrs. Wright killed her sleeping husband because of his destruction of the one beautiful thing in Mrs. Wright's life. Mrs. Peters recalls a boy who took a hatchet to her kitten when she was a little girl. She recalls that at that moment, had someone not held her back, she would have "hurt him."
As the women continue to put the pieces of Mrs. Wright's life together, they comment that she had no children and no real friends. They agree that it must have been a lonely existence for her, especially being married to Mr. Wright. (She had been so much happier before her marriage.) Again, Mrs. Peters recalls an old memory. She remembers that she and her husband had lost their son when he was only two. Mrs. Peters lived away from her own family at the time, and the pain was excruciating because she had no one there to comfort her.
As the time comes for the men to leave and the wives to take the things they have gathered for Mrs. Wright in the prison, the women stand united in their silent agreement not to reveal the dead canary: the probable cause of Mrs. Wright's murder of her sleeping husband. They keep it a secret, hiding it from the men.
At the beginning of Mrs. Peters is a staunch supporter of the male-dominated society of which she is a part (at the turn of the century). However, as she starts to understand Mrs. Wright's experiences, and connect them to similar experiences of her own, surprisingly she changes her mind and joins Mrs. Hale in her silent support of Mrs. Wright.