It is half past midnight and I am at the Edinburgh festival fringe, pretending to masturbate a cucumber in a variety of elaborate ways. Under ordinary circumstances, this would seem like a fairly remarkable state of affairs, but then you don’t know the day I’ve had. I’m into my 12th hour of an audience participation marathon. I’ve spent the afternoon and evening literally running from venue to venue at the world’s largest arts festival, taking part in as many shows as I can that require not just the presence of paying punters, but their active involvement. I have sung a Frank Sinatra song with a woman covered in blue paint. I have sat in a garden shed just off Lothian Street and read the Chilcot report aloud. I have played guitar for the first time in about 25 years, on stage, alongside a rock musician I once interviewed.

I have had genuinely life-affirming experiences and I have felt mortified embarrassment on such a scale that, by comparison, pretending to elaborately masturbate a cucumber in a room full of people seems a mere bagatelle. This state of affairs is compounded by the fact that everybody else in the room is pretending to elaborately masturbate a cucumber as well, this being the climax – forgive me – of the Wank Bank Masterclass show, in which a charming and very funny gay Australian guy called Adam imparts the knowledge he accumulated while working, as he puts it, “giving on-call massages with happy endings to homos and business daddies” in New York City. Some of his techniques are apparently derived from ancient Taoist Tantric techniques. Others, I suspect, are of his own devising: “I call this one Rock Around the Cock.”

I am doing all this because audience participation is a big thing at Edinburgh: not just in what you might call its traditional form – in which a luckless soul is hauled out of the crowd by a comedian who proceeds to make fun of them, to widespread hilarity – but in more complex and intriguing ways. There is an increasing reliance on members of the audience not just to be the butt of a joke, but to use their characters or skills or personal history to actively contribute to the richness of the show. “A lot of the big fringe successes in recent years have had a certain degree of audience participation to them,” says Tom Searle of production company Show and Tell. “There has been a real buzz around immersive theatre. Big shows such as Trygve Wakenshaw, The Boy With Tape on His Face, Dr Brown, Adam Riches, they all used people from the audience in a certain manner, often leaving them with quite a lot of stuff to do, never making them the joke, making them the star for five minutes. Nina Conti, who is now smashing it in the West End, her show is very heavy on audience participation. Part of her ventriloquism act is getting people out of the crowd; she puts masks on them and talks for them, but the joke is never on them.”

One explanation for the rise might be to do with wider trends in popular culture. We live in a world where we expect virtually everything to be interactive to some degree, from the plethora of audience-voted talent and reality shows on TV to commenting on news stories online. Why should theatre and live comedy be any different? But Searle thinks there might be other reasons. “If someone can do it well and control an audience well, deal with the level of unpredictability involved, it garners a lot of credit and praise. If you are an audience member, you’re really rooting for that person on stage; it creates an enormous amount of goodwill for that person, which then reflects on the performer and the rest of the show. It has a very positive effect on a show for an audience. It is absolutely a way of winning an audience over.”

But my first appointment is with an artist who’s using audience participation not out of choice but necessity. Bron Batten has been performing Sweet Child of Mine, which her website describes as “a mixture of theatre, dance standup and an awkward family function” for five years; it has won rave reviews and awards at the Melbourne and Adelaide fringe festivals. The point of Sweet Child of Mine is that Batten performs it with her elderly father: it explores their relationship, not least his incomprehension at her choice of career and some of her more outre performances. But just before she was due to leave for Edinburgh, her father fell ill. Instead of cancelling the show’s run at the Gilded Balloon, she is instead performing it with what she describes as “stand-in Edinburgh dads”.

Backstage, she talks about this turn of events positively, describing how each stand-in dad can bring their own experiences of life and fatherhood to the piece. She is in an ebullient mood – yesterday’s show apparently went very well – but I confess I’m not listening properly. Batten sent me the script a couple of days ago, but I only gave it a cursory glance. Now I’m reading it closely with a sense of mounting horror. It is not that there are a lot of lines, and she is happy for me to read them off the page anyway. The monologues I have to deliver – one of them while Bron writhes around on stage in a puddle of blue paint – are fine, I can cope with them. It’s the other stuff I’m required to do. Possessed of a singing voice that can ruin everyone in earshot’s day, I discover that one section involves me and Batten singing Frank and Nancy Sinatra’s Somethin’ Stupid. Another requires me to stand on stage, alone, at a microphone and tell jokes. The jokes are scripted and they’re meant to be bad – dad jokes – but the microphone, the stage, the spotlight: it feels uncomfortably close to performing standup comedy, something I have no more desire to do than to repeatedly slam my head in a car door.

It turns out to be even worse than I thought. I get through the singing OK, but the dad jokes standup section could only be more like the stuff of nightmares if at the end of it I had been frogmarched off stage and back to my old school, naked from the waist down. The jokes are greeted with deathly silence by a small audience that, as it turns out, has the capacity to get even smaller: midway through, two people get up and leave. It’s not just that I’m completely mortified; I feel guilty. Batten, who is clearly a lovely, very talented woman, has come all the way from Melbourne to do this and I’m causing it to fail. I think about something Tom Searle said, about the unpredictability inherent in audience participation, how performers need to have a kind of selection process to ensure that they get the right audience members on stage: “If they get stage fright and can’t do it, they can completely ruin the show.” You can say that again.

Still, if nothing else, it gives me an insight into life at the sharp end of the Edinburgh festival – “You certainly got the full-on fringe experience there,” muses Batten – and a kind of boggling admiration for the resilience of the performers. Afterwards, I feel like spending the rest of the day locked in a darkened room, weeping softly to myself, but Bron seems remarkably unfazed: “You see? That wasn’t too traumatic, was it?”

I don’t have time to fill her in on exactly how traumatic it was. I’m required at a venue called Summerhall, where a show called Putting the Band Back Together is playing. Based on the true story of Mark Lloyd – a friend of director Annie Rigby and lead actor Alex Elliott, who on learning he had inoperable cancer elected to reform the band he’d once played in – it comes with a twist: at each performance, the house band is made up of volunteers from the audience, who turn up an hour early and get a quick rehearsal before taking the stage.

“We had the idea that people might, if you gave them a bit of advance notice, turn up to the show with an instrument that has been in the loft for a bit, get on stage and do something that might be quite difficult for them to do by any other means. You know, it takes a real concerted effort to put a group together, or even say, ‘Right, I’m going to get a set together and play at an open mic event,’” says Ross Millard, the former guitarist of the Futureheads. Millard wrote and performs the songs in the show, and is charged with getting the daily intake of musicians up to speed. “I think it’s a device that shows the audience that we’re all a bit more than the sum of our parts,” he adds. “Bands are less about individuals, they’re about a collective sound, a collective chemistry.”

Unlike singing or telling jokes at a microphone, I can play guitar – a bit – but I still have a degree of trepidation about appearing in the show. I was never any good at it and, furthermore, I haven’t actually touched the thing for more than 20 years. When I realised I was infinitely better at writing about music than playing it, I stopped playing it altogether, there being few things in life more pathetic than a rock critic who clearly has musical aspirations of their own, their every word somehow flavoured with the bitter tang of thwarted ambition. But the rehearsal goes well: the songs are great, but rooted in chord structures basic enough for even my rudimentary abilities; Millard is endlessly patient and encouraging. The performance goes even better. The play is fantastic – funny, big-hearted, moving and truthful – and the shambolic band of rusty amateur musicians fits perfectly with its central theme, about returning to something you love that adult life has distracted you from. Midway through, I realise that I’m not nervous at all. As we play, the musicians occasionally smilingly catch each other’s eyes: we’re all clearly really enjoying ourselves.

Backstage afterwards, there is a really lovely, warm atmosphere, but Closer by Circa calls. The Australian circus company is one of this year’s big fringe hits; they are packing them in at the Udderbelly. It is an astonishing show that I watch rigid with terror. That is partly because some of it is genuinely terrifying – the Circa performers seem to specialise in the kind of routine that looks so dangerous you can only watch it safely through your fingers – and it is partly because I know I’m going to have to take part in it. There are few things that can make you more keenly aware of your own advanced, paunchy decrepitude than being in close proximity to people who are clearly as fit as a human being can be, and who can perform scarcely believable feats of strength and agility. There are performers up there taking the full weight of other human beings on their heads and shinbones; after years of sitting hunched over a computer, I can’t even stand straight.

As it turns out, it doesn’t matter. Or, at least, I think it doesn’t. When I’m called on stage along with several other audience members, everything happens so fast that I have no idea what is going on. Mostly, I just sit there while Circa do their incredible stuff around me. At one point, I have to dance with one of the female performers. “You’re going to twirl me round,” she says. “Now you’re going to dip me.” I realise she means the ballroom dancing move where one dancer reclines, draped over their partner’s arm. For a split second, I have a terrible premonition I’m going to drop her – her promising career was cut tragically short when a cack-handed journalist caused her to crack her skull etc – but then I realise that she can probably dip herself, without my support. I stagger back to my seat, to a round of applause.

Closer is pure entertainment. My next appointment is something else. The man behind Iraq Out & Loud, Bob Slayer, describes it as “a ridiculous thing about a very serious subject”. In a shed he has erected in the centre of town, people are encouraged to read the Chilcot report of the Iraq inquiry aloud. Everyone does 15 minutes each, and it goes on 24 hours a day until all 12 volumes are done. Hence the shed: it is hard to find a 24-hour venue in Edinburgh. The readings are being filmed. Slayer, who cooked up the idea with comedian Boothby Graffoe, thought it might not work – there was a point, about six hours in, when they feared they were running out of potential readers – but it seems to have captured people’s imaginations. Plenty of famous comedians have turned up to read. People keep coming back and asking to read again.

I can see why. While it is clearly not designed to read out loud – the endless footnotes are a bit of a slog to get through – there is something unexpectedly compelling about the Chilcot report. The section I read is about the sexing-up of the dodgy dossier: arguments between officials about whether it should be journalistic and sensational, or technical, perhaps in order to blind people with science. When I come out, I tell Slayer that I think I got quite a juicy bit. “The amount of people who are coming out of the shed saying that,” he nods. “I thought the actual reading might be a bit of a drudge, but it all seems to be interesting.”

Finally, I head over to Adam and his cucumbers at the Wank Bank Masterclass: my day has been nothing if not varied. I think I’ve learned things: about how hard a gig Edinburgh can be; about the psychological and physical toughness of performers; about how much I enjoy playing music with other people; about the Chilcot report. And I’ve also learned that I am unexpectedly good at pretending to masturbate a cucumber in a variety of elaborate ways: so good, in fact, that I am named best in class, or to be more accurate, Wanker of the Day. Never let it be said that audience participation can’t be a voyage of self-discovery.