A MUGGY August afternoon and the BBC Scotland studio in Dumbarton is a hubbub of activity. Usually it is River City being filmed here but today another gang is in town: Still Game.

The ninth series of the hit comedy, which begins next Sunday, will be the last to grace our television screens. After 17 years (even longer if you count its earliest incarnation in a 1996 episode of comedy sketch show Pulp Video) the young-at-heart pensioners of Craiglang are bowing out.

As I watch filming, the action centres on Navid's corner shop. A monitor shows Isa – played by Jane McCarry – loitering by the door. Behind her head is a newspaper rack with stickers for the only three titles you need to read: The Herald, Evening Times and Craiglang Courier.

A few minutes later Jane bustles off set and pops over to say hello. She has her blouse open and her breasts on show. Well, technically they are Isa's breasts, the flesh-coloured padding with its pendulous frontage prompting a hilarious double take. Talk about hanging out.

It's a humid day and with the temperature nudging 21C, Jane is visibly wilting. And little wonder. Her costume comprises six layers: padding, polo neck, blouse, pinny, cardigan and coat.

That's just the top half. There's also the heavy skirt and thick tights, not to forget a wig and Isa's trademark furry hat – which Jane has affectionately nicknamed "Ted" – atop her head.

Hence the gaping blouse. "Feel the quality of that," she says, thrusting an arm of her pink cardigan towards me. "All man-made fibres," chips in Sanjeev Kohli, who plays shopkeeper Navid, as he saunters over to join us.

How long does it take them to get back into the headspace of their Still Game characters? "Oh, about 30 seconds?" smiles Jane.

Sanjeev, 47, pats the rotund padding beneath his costume. "It's the belly. As soon as it goes on, it changes my whole physicality and suddenly I'm Navid again," he says. "To be honest, people shout stuff at you in the street all the time so you never really stop being the character."

Did they expect viewers would fall so much in love with the double act of Isa and Navid?

"The moment was the live show, wasn't it?" says Sanjeev, referring to the sold-out 21-night run that Still Game did at the SSE Hydro in Glasgow in 2014, followed by a 15-night run three years later. Jane nods. "It was amazing. That was the best job of my life," she says.

The live shows included a sketch where Navid and Isa paid homage to a romantic scene from the film Titanic. "We are on the poop deck of the ship," says Sanjeev. "And you have got 12,000 people in the Hydro wanting these two pensioners to get off with each other."

Jane, 48, laughs heartily at the memory. "They were shouting: 'Get her pumped!'" she says. "It is weird because Navid is married. Anyway, we were meant to lean in closer and closer as if we might kiss and then the curtain would come down.

"The key thing is we mustn't ever kiss and cross that line. But one night the curtain didn't come down. Sanj looked at me and said, 'What do we do?' because everyone in the audience was screaming, waiting for this kiss."

Sanjeev strokes his fake beard. "What did we do?" Jane laughs. "I think you dipped me," she says.

As Jane is called back onto set, Sanjeev talks about how it feels to say goodbye to Still Game. "I don't think I have processed it yet," he says. "I can only compare it to the first time we stopped [in 2007], although we didn't know until afterwards that we weren't coming back.

"But in a weird way it never went away and I've got a feeling it will be like that again this time. I struggle to get my head around the fact that we have done it for this long. I can still remember the first time I had this on and looking at myself in the mirror.

"If you had told me then that I would still be doing this now, I would never have believed it. I'm not feeling sadness yet, but it will kick in."

What will he miss most about playing Navid? "The atmosphere on set and getting to say those lines," says Sanjeev. "The way that the boys [the show's co-creators Ford Kiernan and Greg Hemphill] have written Navid is that he parachutes in and gets the killer line.

"They are very generous that way; they give us the brilliant lines. I know Navid inside out, how he is going to deliver it and react to things. He doesn't always have to do the heavy lifting that perhaps Jack, Victor, Isa and Winston have to, but he gets the funnies.

"I love the way he has evolved. I tend to get to do film parodies, such as One Flew Over The Cuckoo's Nest or Breaking Bad, with Navid as this philosopher-warrior guy. It is fun and I will miss all that."

When Jane returns from shooting her scenes, we finish up the interview in Sanjeev's Winnebago. A few bits and pieces of Navid's clothing, the ubiquitous woollen jumpers, polyester breeks, colourful shirts and ties – each item more garish and retina-searing than the next – hang in the wardrobe.

Jane reflects on bidding farewell to Isa. "I genuinely believe she is real," she says. "Even though the TV show is ending, I feel she will still carry on somewhere out there.

"There is never another job where I will be so comfortable. Look at that shoe. You will never get a shoe more comfortable than that."

Her face grows serious. "What I will miss is the team being together. That's it now. We have been doing this since we were young and have shared this big journey in our lives. We will all still be good friends, but it is not the same as coming in every day to work like this.

"Ford was laughing so hard in that last scene that his moustache pinged off. There aren't many jobs where you would be so happy and comfortable and genuinely laughing as hard as that. I can't imagine ever doing a job like this again. It has been magical."

The term Isa has become shorthand for being a nosey gossip. We all know an Isa … Jane nods. "Everybody does. People don't always recognise me but, when they do, the first thing they say is, 'Guess what they call me?' I'll kid on I don't know and eventually they'll say, 'Isa!' Bless them."

As it gets closer to the end, will they see Still Game out in style? "Do you want to have a party at your house, Sanj?" asks Jane. Sanjeev looks like he is considering it. "That is something we have never done," he muses. Jane is warming to the idea. "We should have a wee party," she insists.

Is this really goodbye? "When you see it, it feels final," says Jane. "It will be emotional and special when the last episode airs."

Still Game begins on the new BBC Scotland channel next Sunday (February 24) at 9pm. Further episodes will be shown on Thursday nights at 10pm