This is, I believe, The Extra, by Micheal Shea. There was a novelette version, first published in the 1987. Later (possibly moved by the apparent relevance for the twenty-first century), Shea expanded it into a novel, which was the first of a series.
Producer Val Margolian has found the motherlode of box-office gold with his new "live-death" films whose villains are extremely sophisticated, electronically controlled mechanical monsters. To give these live-action disaster films greater realism, he employs huge casts of extras, in addition to the stars. The large number of extras is important, because very few of them will survive the shoot.
It's all perfectly legal, with training for the extras and long, detailed contracts indemnifying the film company against liability for the extras' injury or death. But why would anyone be crazy enough to risk his or her life to be an extra in such a potentially deadly situation?
The extras do it because if they survive they'll be paid handsomely, and they can make even more if they destroy any of the animatronic monsters trying to stomp, chew, fry, or otherwise kill them. If they earn enough, they can move out of the Zoo--the vast slum that most of L.A. has become. They're fighting for a chance at a reasonable life. But first, they have to survive.
And the mechanical monsters involved definitely include giant spiders.