Mrs. Minnie Wright is the character whose action, namely the murder of her husband, is the catalyst that causes the chain of events that makes the main theme of the play.
Since Minnie is an absent character, it is the inquiry of two ladies: Mrs. Hale, a fellow country wife, and Mrs. Peters, the sherrif's wife, who realize through their clever and inductive thinking what really went on the night of John Wright's murder.
Although the ladies were in the house to gather some things for Minnie while she is in jail, their female intuition leads them to the so-called "trifling details" (trifles) that, eventually, explain everything.
What they conclude is also partly thanks to the information that Mrs. Hale brings into the play. As a fellow country woman, Mrs. Hale knew Mrs. Wright when she was single. At that time, Minnie's name was Minnie Foster; she was a woman who liked to sing in the choir, and who used to dress quite nicely with ribbons in her head and everything.
Mrs. Hale explains that, after Minnie married John, she was seen less and less. It almost seemed as if Minnie was trapped in her own home. Moreover, Mrs. Hale is quite set in describing John under a not-so-positive light. When the county attorney points out at the dire state of the Wright household, Mrs. Hale hints at it not being necessarily due to Minnie being a bad wife, but because John Wright was a very bad husband to Minnie.
MRS. HALE [Looking about.] It never seemed a very cheerful place.
COUNTY ATTORNEY- No -- it's not cheerful. I shouldn't say she had the homemaking instinct.
MRS. HALE- Well, I don't know as Wright had, either.
COUNTY ATTORNEY- You mean that they didn't get on very well?
MRS. HALE- No, I don't mean anything. But I don't think a place'd be any cheerfuller for John Wright's being in it.
Moreover, we also find out through Mrs. Hale that Minnie simply stopped taking care of herself after a while. This is what helps explain that Minnie's house is in a state of chaos: because it is a reflection of the chaotic dynamics of Minnie and John's marriage.
Yet, it is when the ladies do the ultimate discovery when it all comes together: a broken cage where a canary must have once lived is found empty. Later on, the body of the canary is found, carefully put away..but with its neck wrung. There is no doubt that John Wright, in one of his fits of anger, killed the bird that served as Minnie's only companion. As a result, Minnie snapped and killed John in a somewhat similar way.
We can explain Minnie's reaction as a case of battered spouse syndrome. A woman who is consistently abused and does not defend herself actively is said to be only pretending to tolerate a situation to deflect further violence. However, it is suggested that the person who is tolerating the situation will inevitably snap, not just due to the pent up energy that suddenly is released, but because it is an instinctive defense mechanism to avoid further danger. This tendency to deflect reality also explains Minnie's reaction when Mr. Hale finds at the beginning of the play: Minnie is unresponsive, spaced-out, and not making much sense of the importance of the situation. Moreover, Minnie shows more concerns for her compote and coat than for what her fate will be. It is all part of the same issue: she has suffered so much that she has lost her basic sensibilities.