From time to time, Saturday Night Live (and also Jimmy Fallon properties and, presumably, other variety shows filmed with a studio audience) will do a bit where someone in the audience interacts with the host. Almost always, that person will be another actor, one of the show’s writers, etc., and not an actual audience member.
What happens to that seat before & after the bit? Is it just empty until needed for the sketch? Does the original occupant get temporarily evicted for the bit, and then get his seat back?
I suppose another possibility is that the actor sits in the seat until the bit is done, and then the producer’s nephew or a stagehand with nothing to do for the rest of the show or some other not-real-audience gets to come sit there for the rest of the show.
I have not much experience producing shows, but that’s what I would do. I’d never kick out a paying audience member, even temporarily, but I also wouldn’t want to leave an empty seat up front (bad vibes for actors and audience).