What makes Mrs. Peters in "Trifles" a round character?

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.