I believe I once saw some of this trickery being used at a Dame Edna show on Broadway, which I otherwise loved. I just saw her again, performing in Fort Lauderdale last month, and had another great time.
But at the Music Box theatre in January, 2005, part of her routine was to personally pass a very long-handled basket out over the audience, sorta like a church collection thing, and persuade people to remove their shoes to put into the basket. Then she'd dump the shoes onto the stage and make witty comments as she examined them, before tossing them back to their owners.
My BF & I had front row center seats, our feet literally touching the base of the stage, so we were able to see everything very closely, with the shoe basket passing right over our heads. I slumped down in my seat, because I didn't want her to bring my shoes onstage, being all wet & filthy from the winter slush on the New York sidewalks outside.
But as I looked at the shoes being passed just a foot over my head in the open basket, on their way to the stage, I noticed that all their soles were absolutely pristine. Not wetness, no dirtiness, not even any signs of wear, like they had just come fresh from a shoe store. I watched them closely as Dame Edna handled them right in front of me, and they appeared unworn inside, too, looking absolutely brand new, every one.
And I thought, AHA, those shoes, and likely the people wearing them, are a plant. Otherwise, Dame Edna might have to handle some pretty gross examples of encounters with New York City sidewalk crud. It only made sense. And how would she know where they were sitting, unless those seats had been reserved for that purpose, and possibly some little identifying items placed on their clothing as markers, too. It's an old magician's trick.
BTW, at her Fort Lauderdale appearance, where I again was front row center, there were some empty seats, with no evidence to me of "papering." And Dame Edna used this for a joke (imagine her high falsetto voice):
"Oh, and I see some empty seats, even right up front here. Well you know those are for the subscription ticket holders, and they're mostly from Palm Beach, yes. And they buy these seats here in the Parker Playhouse each year, and then they die. Well, we'll keep these seats open in their memory."
The audience roared with laughter, because in this area it's actually quite true. LMAO!!