AN overcast June morning and on an industrial estate in Clydebank a commotion is unfolding. A police car sits behind a grubby-looking white van to which the rear door has been flung open, releasing a swirling mist of dry ice.

A gaggle of women dressed in feather boas, cowboy hats and deely boppers are being helped from inside. They teeter unsteadily in high heels, sipping alcopops through straws. One is clutching a giant, inflatable penis.

Welcome to Scot Squad. The hit BBC Scotland comedy has newly returned to our screens for a fourth series starring Grado – real name Graeme Stevely – and Manjot Sumal as traffic cops PC Hugh McKirdy and PC Surjit Singh.

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I hear Grado before I see him. Which is remarkable given he's wearing a retina-searing, high-vis jacket. His unmistakable, booming voice carries down the street. Beside him, the more diminutive figure of Sumal can be seen trying to diplomatically soothe the affray.

For those unfamiliar with the concept, the show is a clever spoof that mimics the fly-on-the-wall style of documentaries such as Road Wars and Police Interceptors.

Overseeing proceedings are Scot Squad creator and narrator Joe Hullait alongside director and producer Iain "Noddy" Davidson, whose past credits include Only An Excuse, Gary: Tank Commander and Burnistoun.

The scene is titled "Disco Van" and revolves around a hen party gone awry. Hence the colourful props. Beside me, Davidson gives a wry smile. "It's not often a director gets asked: 'What do you do when the inflatable boaby is in your face?'" he deadpans.

Sumal and Grado (everyone uses his nickname; he's never his Graeme or Stevely) are one of several pairings in a cast that also includes Jack Docherty, Jordan Young, Sally Reid, Karen Bartke and Darren Connell.

The duo's alter egos are chalk and cheese: Singh is strictly by-the-book (he didn't hesitate to give his mother a speeding ticket in one memorable episode), while McKirdy is a larger-than-life maverick (who has no qualms about changing his underpants in the front seat of the squad car).

Yet, once the cameras stop rolling, how much does that odd couple persona mirror the real-life relationship of Grado and Sumal?

Sumal: "Pretty much the same."

Grado: "We are completely different folk in terms of what we like. He likes going to the pictures and I hate going to the pictures."

Sumal: "Grado is very outgoing and I'm more reserved."

Grado: "So, what you see on screen is quite true to life."

Sumal: "But he is more intelligent in real life."

Grado: "Just a wee bit."

And that's how they go on. The pair make a hilarious double act, bouncing off each other and volleying one-liners. It's like watching the unlikely animal friendship between a chimp and puppy.

We've returned to unit base in the Maryhill area of Glasgow for lunch where they – at my request – are thoughtfully attempting to describe each other in five words. "Outgoing," says Sumal just as Grado blurts out "caring" at full volume.

Sadly at this point we're interrupted by one of the crew to take Grado to get his "crawling trousers" on for the next scene (the mind boggles). When Sumal disappears off to the make-up chair a few minutes later, Hullait and Davidson talk through the creative process.

Slough-born Hullait, whose mother comes from Greenock, coined the idea for Scot Squad in 2010 while on placement with the Comedy Unit – the award-winning Glasgow-based production company behind the likes of Still Game and Chewin' The Fat – as part of a Channel 4 training scheme.

"We made a cheap taster tape to show to the BBC," he recalls. "The full development took ages and the first series only appeared in 2014."

The Scot Squad cast is a mix of trained and untrained actors many of whom, like Hullait, got their start in stand-up comedy.

Neither Sumal nor Grado have any formal acting training: Sumal honed his craft in community theatre, while Grado's background is in professional wrestling. On paper it may seem a leftfield combination, but it makes for comedy gold.

Scot Squad differs from other sitcoms in that it is semi-improvised. Hullait is a fan of the improv style of US comedian Larry David on Curb Your Enthusiasm and wanted to capture that essence in his own mockumentary-style show.

Instead of scripted dialogue, the cast are given scenarios by the writers and then, well, that's when the magic happens. If something doesn't quite click, Hullait and Davidson are on hand to suggest a few funny lines to keep the ball rolling.

The only exception is Hullait's breathless and dramatic, scene-setting narration which is fully scripted (he is particularly proud of the line: "Lewis Hamilton may not be from Scotland, but all over Scotland from Lewis to Hamilton, drivers love speed …").

How has filming gone on series four? "It is controlled chaos," chips in Davidson. "It has been good, although the weather is our constant enemy. These guys [Grado and Sumal] are in traffic, so they are on the road which means we are outside and end up with sunburnt faces or soaked to the skin."

With Scot Squad shot as if it is a documentary, the scenes are filmed in one continuous take and edited later. "Our longest take is 24 minutes," says Davidson. "I was actually in that one playing a guy who had locked himself in a bin by accident. But because I was inside the bin I couldn't say cut."

Among the challenges, says Hullait, is ensuring that the show feels fresh and current while neatly side-stepping topical subject matter that could feel dated by the time it airs.

"Last year we were filming during the EU Referendum and then there was the change of [Conservative party] leadership," he says. "Grado's character was to say who he wanted to kiss under the mistletoe.

"His reply was 'the prime minister' but we didn't know who it would be at Christmas time. So, we did different takes with him saying Andrea Leadsom and Theresa May."

It can also mean adopting an often kids' glove approach to serious policing stories. Terrorism is the elephant in the room. Although Hullait and Davidson are sanguine when discussing it.

"If you do something quite serious and then it happens in real life, broadcast has to be pulled and there is that sensitivity," says Hullait. "We want the show to go out and for everyone to enjoy it and not to be offended. A lot of the stuff that has been happening in the news lately isn't funny.

"Although there are light-hearted ways of doing it. Last year we had the chief talking about terrorist targets and how he was one. Then we had him compare himself to the Falkirk Wheel which was [rated] slightly higher than him. That is a nice way of going at it rather than seeing any destruction."

As art imitating life, Scot Squad is often uncannily close to the bone. Indeed the show's makers regularly have officers asking whether they have a police insider leaking stories. "They say to us: 'You must have a mole,'" says Davidson. "The feedback we get is incredibly positive."

That said, many of the tales they have since been told by police are too extreme or outrageous to credibly feature. "People genuinely wouldn't believe it," laughs Davidson.

Lunch is over and the second location of the day is a car park opposite Farmfoods at Baljaffray Shopping Centre on the outskirts of Bearsden (who says The Herald Magazine doesn't take you to all the glam hot spots?). As the crew begin setting up, Grado and Sumal chat about their lives.

Sumal, 36, grew up in the St George's Cross area of Glasgow. His late father Malkit worked in the city's famed Koh-i-Noor restaurant, while his mother Sukhvinder still works as a cleaner at Glasgow University.

The younger of two children, Sumal recounts a tough upbringing. "It was quite rough, especially being an Asian kid in that area due to racism. There was a group of kids who if they saw an Asian guy, that was it. One of my neighbours was beaten up quite badly and put in hospital by them."

He aspired to be an animator, but later gravitated towards writing and directing. In his early twenties Sumal saved up money from a job in a pizza shop to travel to Vancouver where he spent three months working behind the scenes on the TV series Smallville.

That sparked a love of acting and back in Scotland he joined a community theatre group. Sumal was directed by Cora Bissett in a handful of plays and did a stint as a radio presenter for Awaz FM before landing his breakthrough role in Scot Squad.

Married to Mandy, 33, he is based in Renfrew. Among his most recent roles was a part in Mariem Omari's verbatim theatre play, One Mississippi, which highlights mental health issues among men.

Grado, meanwhile, fell in love with performing as a wrestling-mad teenager. The 29-year-old, who is the youngest of three children, hails from Stevenston, Ayrshire. "The tap end," he clarifies.

His father John is a taxi driver and his mother Maureen, now retired, used to work in a hospital kitchen. He still lives in his hometown with his girlfriend Kerry Burns, 27. The couple have been together for 11 years.

Grado first caught the eye of casting directors thanks to Burnistoun co-creator Robert Florence who had watched his homemade wrestling videos on YouTube and shared them with the Comedy Unit team.

That led to an audition for Scot Squad, with Grado also cast in the pilot for The Sunny, co-written by Florence and fellow comedian Iain Connell. Around the same time he was the focus of a BBC Scotland wrestling-themed documentary called Insane Fight Club.

He went on to a role in River City and joined the roster of US-based Impact Wrestling which saw his star rise on the other side of the Atlantic where the series is ranked second largest only to WWE. Grado will appear in the latest series of BBC Scotland sitcom Two Doors Down next year.

Away from work what are their passions? "Macaroni cheese," replies Grado without hesitation. He's apparently a bit of an aviation geek. "I love planes," he enthuses. "I have a passion for flight. I love watching cockpit videos of take-offs and landings, aborted take-offs and landings, bird strikes …"

He pays £1 a month to subscribe to the Glasgow Airport webcam. One of his favourite things is to use the FlightRadar24 website to track flights or watch planes take off on the webcam and then run outside to watch them fly over his house in Stevenston.

"I love eating food, going out for my dinner and wrestling," he adds. "This is geeky but every night before I go to my bed I try to watch one wrestling match. How many have I watched over the years? Millions. I like watching old stuff from the 1980s. I love obscure stuff from Japan."

As for Sumal? He rattles off his own list. "Photography, going out for drives, drawing, watching movies and writing screen or theatre plays."

Earlier, as we drove through the Glasgow streets, a group of teenagers cheered and waved when they saw Grado. Does that happen a lot? "Now and again," he says, with nonchalant modesty.

Sumal laughs. "He gets recognised every single time. I'm always impressed with how Grado handles it because he gets a lot of attention. He talks to everyone, takes pictures and sign autographs."

READ MORE: 10 questions for Scot Squad stars Grado and Manjot Sumal

WATCH: Five of the funniest Scot Squad moments

What is it like to be back in their police uniforms? "To be honest, I feel more comfortable in my wrestling Lycra," says Grado. "This is quite uncomfortable to wear for 12 hours a day. I don't know how polis can do it."

"Aye, it's heavy," adds Sumal.

"The steel toe cap boots, layers of clothing and then having to run about daft?" continues Grado. "Nah, I prefer the comforts of my own leotard."

Scot Squad is on BBC One, Wednesdays, 10.40pm