warm prop
n. A person who appears in the background of a scene in a television show or movie.

Example Citations:
“When you stick to a script, it‘s like you are just a warm prop.“
—Matt Day, quoted in Tony Kiss, “Tell Your Mates About Unpredictable Kiss or Kill,“ Asheville Citizen-Times, January 1, 1998

So, what does the future hold for Simon Westaway? Bill Hughes, who certainly regards Westaway as more than just a warm prop, can see him branching out into other areas of film making.
—Rachel Browne, “My fabled career,“ The Sun Herald, October 26, 1997

Earliest Citation:
Next come the aloof adolescents intent on hanging around the beach like warm props on Baywatch, who can barely manage to carry their own sunglasses.
—Kerry Cue, “Why do we love to live beside the seaside?,“ The Advertiser, January 2, 1995

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