"Initially, I was a fan of the show because I was seeing on the telly the streets where I grew up. It made me realise that there were Scottish people out there doing what I wanted to do. By the time I was 11, I wanted to be an actor. Now, I'm realising one of that boy's dreams."
That's because Robertson, 28, who has made his name in films such as Small Faces and the series Sea Of Souls, is joining the cast of Rab C Nesbitt, which returns for a full six-part series this week for the first time in ten years.
Robertson plays Gash, the wayward son of Nesbitt and Mary Doll (original actor Andrew Fairlie having left acting in the intervening decade to pursue a career in teaching). "I've had a few moments when I pinch myself. Moments when I'm wandering about the set thinking: 'I'm Rab C Nesbitt's son. Is this real?' Rab's tartan rocking chair is as iconic to me, as a young guy from Govan, as the seat in the middle of the Starship Enterprise. And I never once had the guts to sit in it."
When Nesbitt, the Govan philosopher in a string vest immortalised by Gregor Fisher, vanished from the small screen in 1999 after eight series, most thought he was gone for good. However, writer Ian Pattison and producer/director Colin Gilbert decided to revive the show for a 2008 Christmas special. It was so successful – attracting five million viewers and a Scottish Bafta for Best Comedy – that they realised there was life in the old dog yet.
The core of four actors – Gregor Fisher, Elaine C Smith, Tony Roper and Barbara Rafferty – have reunited for the series despite busy careers elsewhere. Fisher, in particular, has been attracting long overdue recognition in films such as Love, Actually, The Merchant Of Venice and BBC's adaptation of Oliver Twist. Iain Robertson is joined by another newcomer, Cora Bissett, who plays Camille, the feisty barmaid in Rab's local.
Like Robertson, Bissett grew up watching the show – if illicitly. "My mum and dad were pretty strict about what we watched, and thought it was a bit rude. I used to go round to my mate's and watch it there. I loved it; it felt slightly dangerous to be watching it. Now, as a woman in my thirties, I'm playing opposite this man in a string vest thinking: 'What would my mum say?'"
On the show, a decade has passed and the characters and their world have moved with the times. Govan has eastern European immigrants and mobile phone mast controversies. Gash is a victim of the credit crunch, struggling with mental health issues. Mary Doll is running her own cleaning business. Perhaps most significantly, Rab has stopped drinking.
"I remember watching an episode (in an earlier series] where he has his stomach pumped," says Robertson. "And as soon as he comes round, he drinks the contents that have been pumped out of him. If somebody drinks to that degree, if you brought the show back 10 years later and they were still drinking, they'd be dead."
But a sober Rab, nursing a glass of orange juice miserably at the bar, changes the tenor of the show. He's transformed from loose cannon to a man trying to do the best by his family – even if his methods are frequently ill-advised, while his sidekick, Jamesie Cotter, remains as reprehensible as ever.
"There's a weird poignancy about it which is quite unexpected," says Bissett. "Rab is teetotal and he really struggles with that. There are scenes where he's staring at this pint, and it's poetic and funny but it's also the struggle of this man that thousands of people will relate to. I think that's the secret of really clever comic writing, that you're playing it for gags, but simmering just underneath are these intricate, touching little stories."
Bissett describes her first day on set like "stepping into a weird sort of dream". "It sounds strange, but what I noticed more than anything else was what great actors they were. Gregor Fisher couldn't be more different from Rab C if he tried; he's this calm, understated, dignified man. Tony is more of a funny guy, but he's absolutely not Jamesie. When people become these iconic characters you think they're probably playing themselves a bit – not at all, I salute their craftsmanship."
Rab C began life in TV sketch show Naked Video in 1986. Soon, he was presiding over a cult sitcom that attracted up to six million viewers (on both sides of the Border), breaking comedy ground which would later grow programmes such as Chewin' The Fat and The Royle Family. Stars from Peter Mullan to Anita Dobson, queued up to guest on it, and unknown hopefuls such as Ashley Jensen and David Tennant (a memorable transsexual barmaid) made early appearances.
The comeback also features an impressive set of guest stars, including John Gordon Sinclair, Taggart's John Michie and David Hayman. "Everyone looks like a kid, there's a little twinkle in their eye," says Bissett. "Everybody's going 'I'm on Rab C Nesbitt!' There's a shared pride in it, it's part of our history. Everyone doffs the cap to Rab C."
With the character of Rab, Pattison and Fisher achieved something remarkable. He ought to be the worst kind of Scottish stereotype, an unreconstructed, foul-mouthed, benefit-scamming drunk. Instead he became a wise fool who shed fresh light on the world around him. While some have slated the show for degrading the vulnerable, many more have applauded it.
"One of the questions I had to ask myself, being from Govan, was: 'Is this offensive to where I grew up?'" says Robertson. "And the answer is no, quite the opposite, the show has a big heart, it captures the hope, aspirations and humour of working-class people. Gregor brings something to Nesbitt which takes him beyond this bottom-feeder alcoholic, you can't help but warm to him."
Having spent his first few days on set trying not to burst out laughing as the gags crackled around him, he realised his role on the show was chiefly as a straight man.
"I've worked with Bill Bryden a lot, and the way he puts it is: you're not a striker in this scene, you're the wing man. My job is not to get the laugh, it's to set it up for Gregor or Elaine or Tony.
"It's an honour to be the one who sets up Rab C's goals for him. For a wee guy from Govan who was never interested in football, that's the equivalent of getting to play for the big team."
Rab C Nesbitt starts on Thursday on BBC2
This article first appeared in Scotland on Sunday, 17 January, 2010