Here is a short paragraph depicting several characters interacting in a scene. It is an example of how one can infer the personality of a person by using the Big Five to interpret their actions. Notably, authors may use contrasting characters in order to emphasize their personalities.
"The two of them quickly took to occupying themselves in the armory. The young woman moved brashly throughout the the room, carrying and handling each ammunition crate crudely. Loud thumps echoed about the room the young woman stacked each crate with discontent, nearly slamming one on top of the other as if to protest such a mundane task. Her footsteps were hurried and heavy as she stomped through the various shelves of unsorted munitions. Plastered upon her face was a rather stern expression with a hint of discontent.
"Sitting atop a crate was another younger man. He appeared to be younger than his companion by a year or two, and where she wore a scowl on her face his was a look of focus and patience. One by one he slipped each bullet carefully into the magazine, the bullets locking into place with a soft click. He took to his work steadily, an air of silence surrounding him as he placed the full magazine into another box before reaching for another. His eyes examined each round with precision, observing any defects and deterioration before firmly locking it into the magazine."
Using the Big 5 to analyze each character, we can infer a few things about their personalities.
The young woman may be low in conscientiousness, as demonstrated by her rather rough way of going about her task. She may also be high in extraversion, as demonstrated by the vigor she puts into her task, although her demeanor may rate her low in agreeableness and high in neuroticism.
The male character may act as a contrast to her. He would probably rate high in conscientiousness while scoring low in neuroticism with his calm demeanor. Such scores may indicate a high level of agreableness as well, although his quiet attitude may put him at a low score for extraversion.
As we can see, this short passage demonstrates how an author can use contrasting characters to emphasize their personalities. They often go even further to include interactions between said characters to establish those differences in personalities.