The Gun in Betty Lou's Handbag (1992)
Betty Lou is a librarian in a very small town. Her husband, Alex, is a police detective who takes her for granted. She is the ultimate mouse. After a gangland figure is mysteriously murdered in town, she finds the murder weapon, but can't get anyone's attention to tell them (we're talking very mousy, here). Finally in a fit of frustration she fires it in a restroom and when taken to the police station insists she is the murderer. It has the desired effect, people do pay attention to her. With hints at social graces from the hookers in her cell block, she's on her way.