Hello. You did not present the story to which this question refers, which prevents it from being answered accurately. However, I will try to help you in the best possible way.
The interactions between the characters can develop the theme of the story in as many ways as possible. Among these forms, the characters can develop a conflict that points to the theme addressed in the narrative, as they can present different perceptions of the same subject, or even, stimulate conversations that directly point to the theme of the story. To determine this, you must be able to identify the theme of the story and see how the characters associate with it and what actions promoted by the characters develop it.