Actor Scott Reid on Still Game, Line of Duty and his new theatre tour heading to Scotland

In one memorable scene, the show's leading men, Jack and Victor (Ford Kiernan and Greg Hemphill), help Mick spruce up ahead of a job interview.

As part of his Craiglang-style extreme makeover, the pair take him to a backstreet dentist where the hapless Mick ends up with a comical set of gnashers that bear an uncanny resemblance to the toothy grin of former footballer Frank McAvennie (sorry, Frank).

The character became an instant hit. Within minutes of the episode airing Reid was hit by a tsunami of emails, text messages and social media alerts. He sensibly turned off his mobile phone and went to bed.

When Reid switched it back on the next morning, his jaw dropped. "I had 3000 new Twitter followers overnight with each notification coming individually," he says. "It was the most bizarre experience."

Further testament to that soaring popularity (as if the tattoo homage, Hallowe'en costumes and Mick-inspired tipples weren't evidence enough) came when fans snapped up 10,000 sets of replica rubber teeth during the Still Game Live 2 shows at the SSE Hydro in Glasgow this year.

Which takes a bit of getting your head around, admits Reid. "It is mad. There is no other word for it."

He is quick to credit Still Game creators Kiernan and Hemphill for the leg-up. "The success of Methadone Mick is down to what Ford and Greg wrote," he says. "They created this incredible character and took a risk bringing a young, fresh-faced character into Still Game."

Reid arrived on the duo's radar thanks to a role in the Paul Higgins and Ricky Ross penned musical, The Choir, in 2015. Reid's friend and fellow actor Julie Wilson Nimmo, who is married to Hemphill, had taken her husband along to the show at the Citizens Theatre in Glasgow.

A few months later, Hemphill got in touch. "I received a Twitter message saying: 'Hiya, brother, I have this part in Still Game. Could you send me your mobile number? I would love to have a chat with you about it.'"

Reid's first thought was that someone was pulling a prank. "It was a rainy day and I was sitting on top of my bed at the house I was living in at Seven Sisters, north London," he says. "I hadn't been getting any luck with auditions around that time."

As Hemphill outlined the role of Methadone Mick, so began a remarkable journey. "I was straight off the phone and on to my dad saying: 'You'll never guess what. Greg Hemphill just called me …'" says Reid, who had to swear his family to secrecy about the part.

"Although my family have seen me in different shows over the years, telling them you are connected to something like Still Game, it was like: 'We're so proud of you, this is the best thing you will ever do …'"

Reid recalls the nerve-jangling moment he went to read lines with Hemphill and Kiernan. "As I pulled up in a taxi, they were sitting on the steps outside Greg's house smoking their e-cigarettes in the sunshine. It was so surreal, seeing the two them dressed in their own clothes.

"When we went into the living room, they instantly turned into Jack and Victor. Ford was sitting there in a pink shirt, while Greg had a huge beard at the time. They looked nothing like Jack and Victor, but if you shut your eyes, it was like watching them walk about Craiglang."

Hemphill announced last month that a new series of Still Game is in the pipeline. Will Methadone Mick be back? For now, Reid is keeping his cards close to his chest.

"The great thing about Mick is that he can disappear or come back, that is the type of character he is," he says, carefully. "He is the kind of guy who could have got a job and disappeared down south for a few months. My door is open. I would love to return to Still Game, so fingers crossed."

After wrapping filming last summer there was barely time to draw breath. Less than a week later Reid was on his way to Belfast to shoot BBC police drama Line of Duty.

He played Michael Farmer, a known sex offender with severe learning difficulties, who is arrested on suspicion of kidnap and murder. With the storyline centred on a potential miscarriage of justice, it was a starkly different role to the cheeky chappy persona of Methadone Mick.

"Polar opposites," he confirms. "The vibe on set was completely different. That was a real learning curve. I had come from Still Game where there was just as many laughs on set when you are doing a take as there was when you're in the makeup truck or on unit base. Line of Duty was fun, but in a very different way. It was fun getting into those really meaty dramatic scenes."

The actor is currently playing the lead role in the National Theatre's award-winning touring production of The Curious Incident of the Dog in the NightTime. Adapted from Mark Haddon's bestselling book, it tells the story of 15-yearold Christopher Boone, who falls under suspicion when his neighbour's poodle Wellington is found speared with a garden fork.

While Boone is a mathematical genius with a remarkable brain, he struggles to navigate his way through the complexities of everyday life.

The teenager – who it is implied has an autism spectrum disorder, although this is never explicitly stated in the play – is uncomfortable with being touched, distrusts strangers and loathes the colours yellow and brown.

When Boone turns detective to solve the mystery of the dead dog and clear his name, it sets into motion a fantastical and often frightening adventure.

The role is one that Reid clearly relishes. "The tour has been incredible and quite life changing," he says. "It has pushed me to the limit as a performer: emotionally, psychologically and physically.

"There is such huge demands in a role like this. The weight of responsibility is there every night, but that is what you want as a young actor. It is one of these parts that you grab and run with."

It involved an element of serendipity too. Reid had originally auditioned for the West End version of the show but was unsuccessful. A few weeks later he got a call offering him the tour instead.

"It was incredibly fortunate because I didn't know that round the corner was Still Game and then Line of Duty. It all fell into place. If I had got the part on the West End then I would never have been able to take on those other amazing roles."

The 25-city tour will arrive in Aberdeen and Glasgow in August. Reid has already smashed the century mark for performing 100 shows and the day before we speak was pressed into last-minute action for a matinee performance in Newcastle. "I was understudy to my understudy," he quips.

He is looking forward to performing on home soil, but before then the tour has to criss-cross the rest of the UK, going as far south as Plymouth before winding its way north of the Border. Reid laughs when I ask him what it feels like to play a teenager.

"It doesn't seem that long ago," he says, reminding me that he's only 23 (he turns 24 in a fortnight). "The ages of 15 and 16 were formative years for me. It was an age when I thought I was a man already, that I knew myself and I didn't need anyone telling me otherwise. I started to build a plan for my life."

He was able to partly draw on those experiences when playing Boone. "There is a lot of similarities because by the end of the play he does start to formulate his own plan. Christopher goes through the same hormonal changes that any 15-year-old does.

"He begins to fight the status quo and people telling him what he can and can't do. Those ages during puberty of 14, 15 and 16 for young men are challenging. Without a positive male role model it can be difficult and Christopher has to do it on his own."

Reid has a lovely manner about him. Thoughtful and articulate, he wears his heart on his sleeve and comes across as wise for his years. Almost every statement sparkles with the effervescence of a young man pinching himself at his good fortune in life.

Born in Glasgow, Reid moved to Paisley with his family aged six. He is the younger of two children and has an older sister, Alex, who is a primary school teacher.

His parents Julie and Brian are foster carers, and Reid's father also works as manager of a resettlement home for alcoholics and drug addicts in the Bridgeton area of Glasgow.

His family, he says, have always encouraged and supported his dreams. Not long after they moved to Paisley, a neighbour told them about workshops being run by the PACE Theatre Company.

For the next decade, Saturday mornings were spent playing football with his afternoons devoted to drama classes. "I was never in the house," says Reid. What was he like as a boy? "I was cheeky, loud and argumentative, but at the same time I always tried to have a smile on my face."

While he paints a childhood surrounded by love, Reid alludes to having something of a tougher time outside the family home. "No money in the world could make me go back and do high school," he says. "It was probably the hardest time of my life. Paisley was a tough place to grow up. It was sink or swim. Fight or run away. That environment makes you able to deal with anything."

Reid typically chooses to embrace the glass-half-full philosophy. "It has set me up to be able to live anywhere. I was living in Tottenham and walking through one of the most diverse areas in the country and not even blinking an eyelid because I had gone to school in Paisley."

In his mid-teens, he faced a crossroads about whether to pursue a career in sport or the arts. "My dad said to me: 'Why don't you pick one? You will have more chance at succeeding if you make a choice. Pick the one you think you will do better in.'"

Acting won out and Reid studied at the Royal Conservatoire of Scotland in Glasgow. "I went there as a 17-year-old thinking I knew it all," he chuckles. "I remember the first day of meeting everyone and thinking: 'There are people here who are not from Paisley or Glasgow.'

"There was a girl in my class from New Zealand and another from America. You would ask someone: 'What have you been doing?' and they would reply: 'I've just been on a gap year to South America,' while I was thinking: 'I should still be in sixth year at school …'"

While Reid has found himself increasingly recognised on the back of Still Game and Line of Duty, he insists fame isn't what drew him to the acting game.

"That is not why I do it," he says. "I always take it as a compliment if somebody spots you because it shows you are doing good work. If they have watched something and connected with the character, then that is the biggest compliment you can get."

He adheres to the age-old mantra: don't believe your own hype. "My mum is very good. She tells me: 'Just do your job and keep out of it.' The hype is something I try to ignore because it is detrimental if you believe in it too much.

"You are only as good as your last job. If I went on stage and wasn't focused on giving the role everything, you never know who is going to see it that night."

Reid lists his passions away from work as sport, music, food and wine. "I'm an avid reader," he adds. "I think as an actor the more stories you read and the more people you encounter, whether in real life or through words, it expands your wealth of knowledge."

He is working his way through the Ernest Hemingway collection. "I went through a stage of reading a lot of American literature and first read a couple of Hemingway's books a few years ago. It is something I've really taken to. He writes such wonderful love stories. It is something from a male perspective which is quite rare."

Work, though, has put the kibosh on romance in Reid's life at this juncture. "I don't have time," he laments. "I'm on tour – my job is my girlfriend at the moment."

He is unlikely to be idle on the work front any time soon. The Curious Incident of the Dog in the Night-Time tour runs until September. As that draws to a close, Reid will put out feelers on prospective roles. He fancies doing a film and would love to land a regular part in a TV series.

Ultimately, says Reid, the goal is fulfilment. "By God, there are a lot of actors who are miserable," he says. "You hear stories of guys at their peak who turn to drugs or drink, you hear those stories all the time.

"Trying to be happy and keep family around, that is probably the biggest aim. There is a moment every day where I go: 'This is the best job in the world.' I'm passionate about it and I've set myself the aim that nobody will work harder than me."

The Curious Incident of the Dog in the Night-Time is at His Majesty's Theatre, Aberdeen, from August 8 to 12 and the King's Theatre, Glasgow, from August 14 to 19. Visit curiousonstage.com