hat could be more disorientating for a comedian than not being able to laugh – for nine hours! – nor elicit laughter from anyone else in the room? I'll tell you. Try all of the above on a sunny summer weekend while pretending it's Halloween. Then force the comics to return to the scene of what one of their number, Phil Wang, calls "a cross between a gameshow and a psychological experiment", and relive the whole ordeal for a man from the Guardian. Is it any wonder Wang, Lou Sanders, Tom Allen and others come across as a little punch-drunk 24 hours after emerging from televised captivity on the smash comedy hit Last One Laughing?

It's the day after filming, at Elstree Studios, the show's Halloween special which sees a unique tweak to its winning formula: contestants must not only refrain from laughter but from screaming. "Which I thought might be a non-event," says Wang, "but it's actually harder. I didn't know I was a screamer. Turns out I am." The participating comics – including paranormal obsessive Daisy May Cooper and that maestro of comedy-horror, Reece Shearsmith – seek not only to amuse each other but spook them too. Which makes sense, says host Jimmy Carr. "Laughs and screams come from the same place. There's surprise in a joke, and there's surprise in a jump scare. Comedy and horror mess around in the same area. It's the same bit of your brain."

Not that Last One Laughing is making serious moves into chiller territory: after all, why tamper with a format that's given Prime Video such a huge hit? With over 2m people tuning in per episode, series one was the platform's biggest ever UK launch. The show later won a Bafta for Best Entertainment Programme. "I don't want to overplay the jump scares," says Carr – who first persuaded the channel to develop a UK version of what was originally a Japanese hit. "There's nothing that would trouble anyone. It's not that kind of show. It's just fun. It's 'event TV' – especially when it's tied around something like Halloween – that's great for bringing people together."

And for bringing comedians together too – the more so now LOL has joined Taskmaster as the shop window for standups making the leap from fringe to mainstream. Alongside national-treasure veterans (Bob Mortimer in series one; Mel Giedroyc in series two), Last One Laughing reserves places for up-and-coming acts – see recent Edinburgh comedy award winner Harriet Dyer here. Getting involved, she says, was "a whirlwind, to be honest. But it was a dream come true." All the participating comics agree, mind you, that you can't treat LOL like a career opportunity. "You can't play it like that," says Tom Allen. "You have to engulf yourself in the moment. And that reveals people's true character. There's something very vulnerable about laughing. We feel very open when we do it."

Few comics are more oversharingly their own self than Lou Sanders, a late casualty (red-carded by Carr in episode six) in series one, now returning for series 2.5. There's a strict embargo on spoilers today, but when I ask Sanders if previous experience gave her an advantage in the Halloween outing, her response does not suggest a triumphant comeback. "I thought I would be better," she says. "But I was immediately on the back foot. And Reece got into my head early on." Then there's the knowledge that millions are watching. On series one, "Joe Wilkinson said to me 'no one's going to watch this,' and I believed him. And then it did well. This time, you know there's going to be eyes on it."

Sanders has been charged with giving me a tour of the LOL house, a bar cum living room cum art deco theatre, like a prefab 1920s ocean liner constructed in a ginormous Elstree hangar. Here is the fruit machine that's used to prompt new challenges. There the stage on which contestants perform their 'joker' party pieces – around which more strictures circulate than you'd expect. ("I thought it would be funny to have a fire on my head," says Harriet Dyer. "But I was told: no fire. Then I wanted long arms, but I wasn't allowed them … ") There are also creepy dolls, just for Halloween. Does it get claustrophobic in here, I ask Sanders? "I wasn't in here long enough to know" comes her doleful reply.

But if Sanders' tactics proved inadequate, others enter Last One Laughing with sophisticated strategies all mapped out. "My tactic was to channel Dame Maggie Smith in Downton Abbey," says Tom Allen. "Very aloof, very lofty. So I channelled everything through my eyebrows, rather than a lot of people, who try not to laugh by biting the inside of their mouth." Wang had a more attacking approach. "My mantra was: a good defence is a good offence. Get on the front foot, try to get to people. Focus on those you know at first, because you know what makes them tick. And while doing that, watch the people you don't know, to see what makes them laugh."

Dyer entered the house with an idea of what to do. "I think Sam Campbell said on series two that he thought about, like, kids dying to stop himself laughing. So I thought: should I focus on the worst thing that's ever happened to me? But I'm not sure watching someone be traumatised with what life's thrown at them makes for very good telly." Campbell is the maverick Aussie comic who tested the show's conceit to breaking point last time around, by staying po-faced from start to finish. He eventually lost in a tie-break to David Mitchell. "We nearly broke David," says Jimmy Carr. "At the end he was going, 'I don't think I'll ever laugh again. I've lost that part of myself for ever'."

Series two's other breaking point was reached when – great controversy, this – the comic Amy Gledhill was red-carded by Carr for smiling rather than laughing. My own household can't have been alone in its outrage at this disregard for the show's core conceit. But Carr is unrepentant. "I mean, we'd got to that stage of the day," he says: something had to give. "And it's that water cooler thing: as long as people are arguing about it and having fun, so much the better."

To Carr, the show is as much sports as entertainment: the key, he thinks, to its success. "There's a narrative to sports, whereby you've got to watch to the end because you don't know what's going to happen. And that's the same thing with this show: it's competitive, and you want to know who's going to win." "And each episode ends on a really effective cliffhanger," adds Phil Wang, "which is quite hard to achieve with comedy."

The other clincher, for Carr and the comics, is how rigorously LOL is produced. Says Carr, "the other countries don't do the set pieces we do": the guest appearances, the challenges, the props and tricks contestants can bring to the table – much to Lou Sanders' dismay. "Reece Shearsmith is Mr Halloween, isn't he? He's got all that stuff at home! It's not fair. I haven't got a severed hand at home, have I?"

According to Wang, "shows that prepare more than they need always prosper. And this show definitely prepares more than it needs." Every contestant has a camera trained on them throughout, and a dedicated "spotter" hanging on their every smirk. "I presented it in the room yesterday for nine hours," says Carr. "But I haven't seen it all yet. When you get in the edit, there's a whole other show there. There's all the conversations people have on the screens I'm not watching. I remember on series two discovering all these incredible moments with Diane Morgan we just didn't know were there."

The lineup has already been announced for LOL series three, which broadcasts next year. Harry Hill, Katherine Ryan and Lee Mack all feature. What advice – or warnings – would the Halloween participants impart? "You end up in there for so long," says Wang – lucky for some! – "that your mood goes from giddy and excited to paranoid and nervous. I was doing a lot of pinching, a lot of breathing, a lot of meditation and mindfulness." It was, says Tom Allen, "an intense experience, trying to withhold two emotions and not let them out of the bag. It's exhausting."

But it's all in the service, says Allen, of something much needed in our not-always-joyful times. "The series speaks to a childlike sense of play, one that reminds us of being in assembly or in class at school and not being allowed to laugh. There's nothing more exquisite than that burning desire to laugh while having to keep a lid on it, the steam cooker pressure that brings. Last One Laughing returns us to that youthful joy. I think it's something people really crave."

Prime Video from Friday.