Jack Lowden, a first year student at the Royal Scottish Academy of Music and Drama in Glasgow, beat off competition from hundreds of hopefuls to play the all-singing, all-dancing Billy in the ad.
And the 18-year-old made his national television debut as the advert aired for the first time on ITV Scotland to millions of viewers during the half-time interval of the crunch Champions League clash between Arsenal and Manchester United last night (Tuesday).
Jack told the Border Telegraph: 'It was such a relief to find out I"d made it. When I went to the auditions, we were split up into groups so I had no idea whether I was in or out. It"s great to be part of it. It"s Irn Bru; you have to do it. It"s like national service.'
The campaign, which has already proved an online hit and been viewed thousands of times on video-sharing website YouTube, is a spoof take on Disney"s High School Musical.
And the former Earlston High School pupil, whose character resembles original star Zac Efron, said the experience of filming was very different to the stage acting he was used to. He began his acting career with roles in the school"s annual productions and then with Galashiels Amateur Operatic Society where he played the lead role in The Boyfriend.
'It was a lot more technical than acting on the stage. You"d be given a spot to finish on and you had to finish on that spot or they couldn"t get the shot they needed. They were long days, but it was so much fun on set and we had a really good relationship with the crew. It was an amazing experience.'
Jack, who is studying for a BA in Acting, added: 'I think the next thing for me is to get back to the academy and get this degree. If other work was to come along, that would be great, but that is my priority. After that I hope to establish myself in theatre. Then, who knows?'
Jeff Thomson, principal teacher of music at Earlston High School and Jack"s former musical director, said: 'Everyone at the school is delighted for him, and so proud. Jack works really hard, he has confidence and he has ability, and he"s shown why he was good enough to get into the RSAMD. The world is his oyster.'
The advert was created by the Leith Agency and the total campaign cost around £2million.
Jonathan Kemp, A.G Barr Commercial Director, said: 'The idea behind this campaign was to create something that celebrated Irn Bru and Scotland. We wanted to do this in an Irn Bru way - but at the same time take inspiration from a number of sources such as Grease, Gregory"s Girl and Hairspray.
'The outcome is a really entertaining mix of quirky characters and down-to-earth Scottish humour. This is all brought to life by some really talented young Scottish drama students who I"m sure will give people something to smile about.'