What term describes characters that fill out a story without significantly influencing it?

The term that best describes characters who fill out a story without significantly influencing its plot or main events is stock characters. Stock characters are stereotypical and often lack depth, serving specific roles that are easily recognizable in storytelling, such as the "dumb jock" or the "wise old man." They usually do not experience significant development or change throughout the narrative and are typically used to support the primary themes or action without driving the story forward.

In contrast, round characters are fully developed and complex, often playing a critical role in the narrative. Minor characters, while they may also be less central than protagonists, can have varying degrees of influence and development. Protagonists are the main characters around whom the story revolves, significantly impacting the plot and other characters. Hence, stock characters specifically embody the notion of filling out a story while having minimal impact on its overall trajectory.