Chancing his arm, the workie tried to tap Robertson for a couple of freebies. Robertson turned the tables on the guy, and asked if he too could supply his services gratis.
"Oh, but that'll cost you," the disgruntled man said.
Robertson pointed out that it was the same for him, and that, just because he was an actor, he wasn't rich, and was afforded no special privileges at the box office. Even if he was, he certainly couldn't afford to let anyone and everyone who crossed his path have access to his wares for free.
As Robertson prepares to revive his solo turn in Ronan O'Donnell's play, Angels, as part of the Traverse's Edinburgh Festival Fringe season, such an incident says much about Robertson's approach to his craft.
On one level, his on-stage charisma and sheer chutzpah when inhabiting a role inspires an unshakeable watchabilty he's been aware of ever since a teacher pointed out his aptitude for drama when still a schoolboy. On another, there's a down-to-earth pragmatism that recognises the ups and downs of earning a living in the spotlight during hard times. For all the attention Robertson gets, sometimes what he does is just a job. At other times, as with Angels, it's most definitely not.
"For me it read almost more like a short story than a play," Robertson says of his first encounter with O'Donnell's script about a security guard's Kafka-like incarceration for a murder he may or may not have committed.
"But my party piece is doing Tam O'Shanter, and Ronan's writing's dead poetic as well, so I started seeing the drama in it. Then when [director] Graeme Maley said that there's not going to be any set, but that it's going to be really spare, that sounded exciting as well.
"I'd not long read Peter Brook's book, The Empty Space, and Angels is very much in that mould of what Brook was talking about, how theatre should be done. I'm not a big fan of one-man shows to be honest. After about 10 minutes I'm usually thinking: 'Wouldn't it be great if another actor came on?' But Angels feels different."
Having last appeared on-stage in a revival of Tom McGrath and Jimmy Boyle's 1970s interpretation of former Glasgow hard-man Boyle's road to Barlinnie, Robertson seems already attuned to the themes of O'Donnell's play.
"I'm fascinated by this presumption of guilt over innocence," Robertson says of Angels, first seen at Oran Mor. "I don't know whether Andy Coulson would necessarily enjoy the play."
The character Robertson plays is no Andy Coulson, however. Rather, Nick Prentice is a loner whose off-duty scribblings appear to nail him for the crime, but who has an awful lot more going on inside himself than he might initially let on.
"When you first meet him, he's a guy who's been in a cell for a few hours, so he's full of nervous energy and he's incredibly anxious," Robertson explains.
"He seems a decent enough guy, but whether he's innocent or not, he's carrying round with him a burden of guilt. Obviously, I carry a burden of guilt round with me as well, but that's because I'm a Catholic. Nick's guilt isn't about that."
Robertson sounds wise beyond his years when he says this. This may have something to do with the fact that, despite only being 31, and being best known these days for taking over the role of Rab C Nesbitt's son Gash in Ian Pattison's Govan sit-com, Robertson has been acting professionally for the best part of 20 years. That was when, spurred on by his teachers, the then 11-year-old joined a local drama group.
"I'd always been really into doing it," he says, "but I still remember going into the community centre feeling really scared and not knowing what to expect."
Somewhat precociously, Robertson applied for and won a scholarship at the Sylvia Young Theatre School, a famed breeding ground for soap-opera stars , boy and girl bands and the late Amy Winehouse. He was 12.
"If I'd waited till I was 18 it wouldn't have happened," he says.
"I was naïve at that age, and if I'd had the time to think about it, I wouldn't have done it. But being in that room, the emotional release I got from doing improvisations, after 20 years, I still get that. It never gets tired for me."
Robertson was spotted by film director Gillies Mackinnon, who cast him as the youngest of three brothers in 1960s Glasgow in his 1996 feature, Small Faces. It won the by now 13-year-old Robertson a Scottish BAFTA. A stint in Grange Hill followed, as well as a study of the perils of teenage drinking in a health education ad.
"The strange thing is, I kept Grange Hill off my CV until last year, because I was embarrassed by it. I was really young, but the way I was cast, I was playing the Scottish bad guy, and doing the Grange Hill equivalent of what Gerard Kelly did in Brookside. It took me 10 years to realise that I'd been in great things like Kavanagh QC and Grange Hill, because I was just too young to realise."
As young as he was, Robertson's antennae for good roles was razor-sharp.
"I'd met Gillies McKinnon by a fluke," he says, "and while I was waiting to hear about Small Faces, I was offered a part in a kids TV programme called The Demon Headmaster. For a 13-year-old, going on telly sounded like a great idea, but for some reason I held out until I got the film."
As a grown-up, until Rab C Nesbitt, Robertson's highest-profile appearance was in Sea of Souls alongside Bill Paterson and Dawn Steele.
Despite living in London these days, Robertson has been seen more on Scottish stages than elsewhere over the last few years. He appeared in the Traverse's revival of John Byrne's Slab Boys trilogy in 2003, and played Romeo at the Citizens.
The theatre experience he remains most proud of, however, is Bill Bryden's turn-of-the-millennium revival of The Mysteries at the National Theatre.
"There's something really interesting going on in Scottish theatre at the moment," Robertson says, "and I think that's something to do with the amount of Scottish talent that's around."
With this in mind, Robertson's ideal role is a typically audacious one.
"Richard III," he says in a flash. "I'd love to do it Scottish, and have it set in Scotland.
"Normally there's this whole idea of him walking onstage with a hump, but there's nowhere in the text it says that. I think I'd have a really interesting take on things. It's the same with Angels.
"The first time I did it word-perfect, I came off-stage and was nearly crying. When art creates a moment that you only normally get when you go to Glencoe or somewhere where you can marvel at the majesty of nature, that to me is a very special thing."
Angels, Traverse Theatre, to August 26, various times www.traverse.co.uk.