Boomtown employs hundreds of actors. They roam the fictional city in character all weekend, improvising street scenes, flogging imaginary products, refusing to break even when you ask them where the toilets are. What the festival doesn't know is that it once had the frontwoman of Panic Shack on the books. "I've actually worked the festival once upon a 2017, I think it was," confesses Sarah Harvey. "I was doing a walkabout with some friends, which is essentially improv acting in costume. It got me a free ticket - and yeah, it was interesting to say the least."

Nine years on, the costume department's loss is the line-up's gain. Panic Shack arrive in the Matterley Bowl this weekend to play not once but twice, first on NEXUS and then again over in the punk district at Hangar 161, because one dose of Cardiff's rowdiest is never enough. As returns go, it could be worse. "Boomtown is such a cool festival," Sarah says. "I've always loved the storyline and how much effort goes into making the stages and areas, so impressive!" She has one item on her own wish list: "I'd love to catch Princess Nokia, never seen her live but love her!"

We catch the band in classic condition. Em Smith is prepping a weekend of DJ sets; Sarah is, in her own words, "hungover AF but dragging myself outside to be in the gorgeous - and scary because it's going on forever - weather". It's been that sort of year. Twelve months ago, their self-titled debut crashed the Top 40 and went to Number One on the rock chart. Since then, there's been Jools Holland, a Welsh Music Prize nod, festival sets that never fail to go off, and - the big one - June's first-ever North American headline tour. "We're international now!" beams Sarah, "[it] feels like our world has opened up."

America taught them things. Important things. "We basically learned about ourselves that we're just fucking class at making music and we were born to sit in a van for 12+ hours a day," reports Em. "We learned that you can perform the same set 100+ times and still not remember the order of the songs for the set list, will most likely leave the set list in the green room anyway, and that we will eat anything dill pickled that we find in the gas station."

The rooms have grown all year, and the crowds have grown teeth. "As the crowds have got bigger and we've reached more people, they've definitely got rowdier and diversified a fair bit," says Sarah. "We have so much love for the 6music daddies, but we also absolutely fuckin love when crowds put their phones down and start a mosh pit, and we've been seeing that loads more."

Festival crowds are their own sport, mind, and Em has the game figured out. "More often than not, people dunno who the fuck you are - like with festivals, I guess it feels like we're trying to win people over. I always feel like we've nailed it when I see punters' faces go from looking confused like they dunno what's happening to smiling like they're in on the joke with us."

This year's Boomtown chapter is themed around community, connection and collective joy, which turns out to be something Panic Shack have strong opinions about. What makes a crowd a community rather than a room full of strangers? "I think people being respectful of what a good time means to each individual is integral," reckons Sarah. "Helping people up if they fall in the pit, catching people when they crowd surf, shutting up when you should shut up - screaming when you should scream! Just not being a dickhead, I guess."

There's new music in the arsenal, too. May's 'grin & bear it', their first release since the album, was rescued from the reject pile and rebuilt with Ross Orton until it hit hard enough to earn a place, and the band's quality control remains refreshingly unscientific. "We either all know it's a banger or we're all like naaaaa, that's not the one, and put a pin in it till we're feeling inspired by it again," says Em.

The song itself was born on a lonely night shift, which points to the thing that makes Panic Shack's rise (like that of most other bands 'these days') both heroic and slightly enraging: they are still, all of them, holding down day jobs. "Not gonna lie, it's a little stressful," admits Sarah. "Being away for months on end, touring America and playing festivals is class, but running out of holiday pay and scraping money together to pay your bills… the harsh reality of the after-tour comedown."

Ask what they're hungriest for, and the answer arrives without a beat of hesitation. "Absolutely starving to hand in our notices together and quit the day jobs," says Sarah. "That'll feel truly exhilarating and a long time coming. People from our backgrounds don't usually get to do this job, and for it to be our full-time job, that'll be pure magic."

Until then, the calendar is doing its best to force the issue. After Boomtown comes Green Man - "which we are obvs buzzin' for," says Em - then Portugal's Meo Kalorama, "then we're back to the states to eat more pickled goods and support Sex Pistols". And after that? "The UK may or may not be getting a good seeing to in the winter, so keep your eyes peeled for that!" You heard it here first, in the vaguest possible terms.

Through all of it, the love of the punchline keeps Panic Shack going strong. Their whole world - the videos they shoot themselves, the between-song patter, the general air of a gang mid-caper - isn't put on, because it couldn't be. "I don't think there's a single moment when we're all together that we're not laughing and joking, so what you see is not an act,"says Em. "We all find each other hilarious, and I guess that comes across in what we make. I think that people could, and do, call us many things, but you're probably not calling us boring."

Boring has never once been on the charge sheet. This weekend, a fictional city full of professional characters is about to meet four of the realest ones in Britain, and one of them used to be on the staff.

Panic Shack's single 'grin & bear it', and their self-titled debut album, are out now. Boomtown takes place at Matterley Bowl, Hampshire from 12th-16th August; visit boomtownfair.co.uk for more information.