The Extra by Michael Shea.
People, including teenagers, are hired by movies to supply live-action violence against each other and animatronic spiders because they're so poor this is the best deal available to them.
I've read what I didn't realizes was an excerpt in a Shea collection called Polyphemous, which more or less horror in various genres, all of it pretty good.
This entry was posted at http://nancylebov.dreamwidth.org/536033.html. Comments are welcome here or there. comments so far on that entry.