Like yin and yang, Thelma and Louise are two opposites that form a pretty awesome whole.
Thelma's the zany naive counterpart to her pillar-of-strength-and-updos pal, Louise. She doesn't start off particularly well. What we first see of Thelma's house, for example, is a sort of DIY half-done wallpapering job. We also see Thelma eating a granola bar or diet bar or something in tiny bites over the first half hour of the film. Space cadet? Maybe a little. Bored housewife? For suresies.
Thelma acts like a child a lot of the time and is constantly challenged by Louise. Early in the movie, for example, Louise even asks her, "Is Darryl your husband or your father?"—which pretty much sums up that dysfunctional relationship.
But despite her childlike tendencies and girly look, Thelma just wants to have fun. And for her, fun means getting out from under the oppressive paws of her long-term partner Darryl Dickinson—who, in personality, totally lives up to the first syllable of his last name.
Sometimes that fun gets Thelma into trouble. In a way, a lot of what happens in the movie is a result of the fact that Thelma is just so happy to get away from her husband and everyday life that she doesn't think about any consequences. Trip to the bar? It's just the beginning.
However, even though Thelma can come across as sort of a helpless daisy most of the time, she's got a rebellious streak that ends up being a spirit-saver to Louise in the end. That streak also gives Thelma the sass she needs to deal with a whole pile of jerks (and cash registers) along their journey.
We get a hint of Thelma's inner grit when she "leaves Darryl stuff to microwave" instead of asking his permission to hit the road with her best friend, for example, and we see it in full play by the end of the movie, when she somehow manages to disarm a cop and lock him in the trunk of his own car. Her response to his measly pleas for mercy because of his wife and kids? She tells him to be sweet to his wife: "My husband wasn't sweet to me and look how I turned out."
Yeah, Thelma transforms over the course of the film, and she ain't turning back, folks. One taste of freedom gives her the realization that she'd rather die than go back to her old life.