Update 21/5/13. After this piece was published I was contacted by Chris Cox, self-styled "mind reader who can't read minds" and we had a very interesting chat about the use of plants in magic and mentalist shows. My interview with him appears in italics on the next page after the end of the original article (click on "next" at bottom right of this page).

"It was obviously a plant." If I had a pound for every time someone has said this to me after a show that involved audience participation I'd have enough money to stage my own Edinburgh Fringe Spectacular. In recent years audience participation has become part and parcel of comedy shows and quite often these are the parts that make the show so memorable, so perhaps it is no surprise that punters think it is all scripted.

When I judged the Foster's Award last summer the panel had some heated debates about Dr Brown, whose show to some extent hinged on the comic skills of a member of the audience. Even though we had seen the show twice – once to put it on the shortlist, then again to judge the winner – some panellists were convinced that Brown was using stooges. Yet we had seen the show on different nights, which would mean he would have needed about 30 willing, well-versed, equally skilled different volunteers.

Eventually the panellists who championed Dr Brown convinced the others that he was bona fide and he won the Foster's Award. I hoped we were right and the contributions were spontaneous. As it happened I saw the same show again at the Soho Theatre earlier this year and this time, by bizarre coincidence, the person picked out was sitting with a friend of my partner. So the next day I asked her to investigate and find out once and for all if it was a set-up…

The answer was more illuminating than I expected. He wasn't a plant, but he did happen to be an actor. Which might have explained why he was so good at following Brown's directions, which included riding an imaginary bike and, well, how can I put this delicately, miming being fucked from behind by the performer. After the show Brown apparently thanked him and joked: "You make it difficult for me, you were so good people are going to think you are a plant…"

This is a problem of audience participation. If it works out successfully cynics will think it has been planned, if it doesn't work then what is the point? Luckily most audience members are not cynics and go with the flow. Performers such as Al Murray, Adam Riches, Nick Helm and even shy-retiring James Acaster have involved civilians with terrific success.

Performers who use punters are very clever. The banter at the start that might seem a bit like a throwaway warm-up routine is often a way of sounding people out to see who might be a good sport and who might not want to play ball. That's why the person that ends up being hauled onstage is often well up for it. The other useful thing about the process is that getting someone up unites the rest of the audience – once someone is walking up  to the stage you can feel an entire theatre unclench its buttocks simultaneously.

It is rare to see someone onstage who does not do quite what is expected of them, although that was what happened at a recent Jimmy Carr gig when two fans were handed scripts and had to act out a romantic sketch. The woman initially said something on the lines of "this is rude, I can't say it" but as the sketch went on her inhibitions seemed to fall away. She did a sexy dance and upstaged the man while he was trying to deliver his lines and ended up in a clinch with Carr. The show was being filmed for DVD – it'll be interesting to see what makes the final cut. It was funny but not what was expected. I would hazard a guess that she definitely wasn't a plant.

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