At the beginning of the story, Mary is a docile, loving, subservient housewife who lives to please her husband. She thrives on routine, stability, and making sure that her husband his happy. Her personality is also not very evident separate from him; she doesn't seem capable of independent thought, or making decisions on her own without his okay first.
After "the incident", some unexpected aspects of her character come out. Instead of weeping in despair as her entire world crumbles around her, and confessing her brutal crime of murdering the man she worshipped so much, she calmly takes action to cover her tracks so that she can continue on without him. Her first lucid thought is, "All right...so I've killed him," not hysterics. She puts dinner in, does her hair, practices her smile, comes up with a plan. She is making all sorts of decisions on her own, calmly, all ensuring that she can live an independent life without him. She has the audacity to feed the cops lamb, and the confidence to have them in her house for so long, lying the entire time.
In short, she changed from a docile, meek, seemingly clingy woman incapable of independent thought to strong, lucid, independent, capable woman. The real question is did she change at all, or did the circumstances bring out traits that had lain dormant? Either way, the Mary that emerges from this story is much more strong than the one we see originally.