If we stick to the general rules that apply to all films, an extra should never look into the camera lens.
This is perhaps the most important rule, because if the extra doesn’t follow it, she can ruin a take and cause big problems for the production.
Imagine what would happen if the director realized afterward, or even during editing, that one of the background extras was looking into the lens…
Therefore, a violation of this rule generally results in the immediate dismissal of the offender.
Two other known rules concern the selection of extras before filming begins.
While extras must bring their own clothes to the film, they won’t be hired if they wear something too flashy or a T-shirt with a very legible slogan. They should blend into the background, not stand out.
Likewise, an extra who vaguely resembles one of the actors on screen will not be hired. Nothing happening in the background should monopolize the viewer’s attention.