What characteristic best describes background actors?

Background actors, often referred to as extras, play a crucial role in establishing the environment and atmosphere of a scene without participating in the storyline through dialogue. Their primary function is to support the main action, contributing to the realism of the movie without having any speaking lines. This allows the focus to remain on the lead characters and the central narrative, while background actors help to fill out the scene, creating a more immersive experience for viewers.

The other options do not accurately characterize background actors. Prominently featuring in the plot and having many speaking lines are attributes more aligned with principal actors. The notion that they typically play villains is also incorrect, as background actors can portray a variety of roles that aren't limited to antagonistic figures. Their main role is to enhance the film's setting, which is best encapsulated by the idea of supporting main action without engaging in dialogue.