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The 54-year-old arrives in Shieldinch as Sam Spiller, the long-lost father of Stevie O'Hara, in Monday’s episode.
For Grant, landing the role brings a certain amount of validation. For decades one of the best known voices on local and national radio and with 23 King's pantos already under his belt, Grant may well have established himself as one of the Capital's favourite entertainers but back in the day, when he decided he wanted to train as an actor, the drama schools didn't want to know.
"In the Eighties, I was that boy at school who wanted to act. All through school, I was set on going to drama college and I still remember the shock of not getting in. When all the drama colleges knocked me back it hurt, but not getting into Queen Margaret hurt the most because I got so close; having been told I was unsuccessful, a lecturer took me aside to say it had been between me and this other guy... and that they had gone with him.
"I remember thinking, 'I don't know if that makes me feel better or worse. What am I going to do now?' In my head, it was all I was ever going to do. Thirty-six years on from those failed auditions, I have finally got a job and an opportunity to make it happen."
Raised in Morningside, Grant eventually joined the police force before carving a niche for himself as a presenter on local radio and TV, but he admits that the prospect of stepping onto the River City set for the first time terrified him, even though he had often discussed the possibility with his best pal, the late Andy Gray, who played Pete Galloway in the series.
He laughs, as he reveals, “We talked about how I could maybe get on River City as Pete Galloway's much better looking and much younger son."
More seriously, he continues, "But Andy's legacy on the show is huge and it has to be said he had no small part in this opportunity coming my way."
He explains, "It happened really quickly. In March I got an email from the series’ producer, Martin McCardie. A number of us had done a Facebook tribute to Andy, and Martin was one of the producers. In the email, he said he'd watched my interview about Andy, in which I'd recalled how much he had mentored me, and that there was a part coming up in River City they thought I might be perfect for.
"I imagined it would be something to do with Andy's character; maybe I'd come in to do an episode or two to tie up the loose ends of the Pete Galloway story, the one thing Andy never got to do. But it had nothing to do with that.
"Martin had emailed on the Wednesday and we had a Zoom on the Friday, by the end of which he’d offered me the part. I'll never forget sitting in my wee home-made studio at the back of the house trying to remain calm and keep a degree of dignified professionalism when all I wanted to do was run about screaming the house down.
"The most surreal, wonderful, emotional two or three days that I can remember followed. A culmination of everything Andy and I had done together at the Festival and beyond led to this.”
Watching clips of his first scenes, Grant admits he could "see the fear" in his eyes.
"I remember how scared I was the first few days," he says.
Perhaps surprisingly, he reveals that one of the main things he had to get over after being cast was a lack of confidence.
"Stick me in a radio studio, in a room of 500 people or on a panto stage and I'm fine, but walking into a TV studio was alien to me, it was an environment so far removed from my comfort zone that my confidence was at the level of the new boy at school.
"My first day on set was a week or two before I started filming - I went through for a costume fitting. It wasn't being used at the time so I was pretty much on my own on Montego Street. After 20 years, it's an iconic show, so that was quite an overwhelming moment.
"When I went back to do my first scenes it really was all about getting my confidence up and not being too self conscious but everyone was so welcoming that my insecurities and fears abated after the first couple of days."
A life-long Hibby, the soap's script-writers were quick to give a nod to Grant's green tendencies in his first episode, all of which helped him settle as did working with Iain Robertson, who plays his son, Stevie.
The actor laughs as he reveals, "There's one line that made me laugh out loud when I saw it, but none of that was my doing, that was the wonderful writers putting things in to help me.
“Actually, I'm very fortunate getting a role like this. When I first read the script I thought, 'This is a wonderful opportunity to stretch my acting chops'. This is new territory for me, so getting to work with Iain Robertson, who has been there, seen it, done it, has just been terrific. We had a couple of Zooms with the director before going on set, then it was like, 'How are you doing? Right, shall we do this?' and boom, we were straight into it.”
Come Monday, Grant will be settling down to watch his debut with the family, albeit through his fingers.
"They're absolutely stoked and champing at the bit to see it when it airs, particularly Lori, my daughter, who is studying theatre at York just now and is already miles ahead of me in the acting game. She has already done a series of The Demon Headmaster for CBBC, so I've had to up my game a wee bit."
River City, BBC Scotland, Monday 10pm, BBC One Scotland, Tuesday, 7pm and on demand on BBC iPlayer