Because "Benjamin Button" is told to us as a sort of tall-tale by its first person narrator (see "Narrator Point of View" for more), it is fitting that we get a fair amount of character information from him directly. It is the narrator, for instance, who explicitly tells us that Benjamin is "by nature obliging" and that "it was all part of Roger Button’s silent agreement with himself to believe his son’s mortality" (1.3.7, 1.3.17). The details of Benjamin’s changing marriage to Hildegarde are similarly, explicitly revealed.
Actions speak loudly in "Benjamin Button." Roger Button forces his son to shake a rattle at intervals when he is a 70-year-old baby, makes him wear child’s clothes, and even tries to make him drink warm milk. His insistence that Benjamin be normal reveals his deep-seated need for conformity and social acceptance. Similarly, Benjamin’s easily obliging nature is revealed through his own actions: he plays along with his father’s delusions. We see that he shakes the rattle as he’s told and even "contrive[s] to break things around the house" because that makes his father happy (1.3.7).
In "Benjamin Button" it is abundantly clear that our physical age determined our character. Consider the passage about Benjamin and Hildegarde as they grow past each other:
In the early days of their marriage Benjamin had worshipped her. But, as the years passed, her honey-coloured hair became an unexciting brown, the blue enamel of her eyes assumed the aspect of cheap crockery—moreover, and, most of all, she had become too settled in her ways, too placid, too content, too anemic in her excitements, and too sober in her taste. As a bride it been she who had "dragged" Benjamin to dances and dinners—now conditions were reversed. She went out socially with him, but without enthusiasm, devoured already by that eternal inertia which comes to live with each of us one day and stays with us to the end. (1.7.7)
The fact of Hildegarde’s aging physically is an indication of the changes to her personality, tastes, inclinations, and her entire character. Similarly, as Benjamin is physically younger, his character is now that of a younger man.