Now, who would have guessed that, 13 years on, Barbara and Sean would be set to appear as lovers in another Boyle romantic comedy, Lark, Clark And The Puppet Handy, the story of a once-successful variety act Lark and Clark, who were also once lovers.
But now, Barbara and Sean are real-life partners, who married seven years ago.
"I could never have dreamt this would happen in million years," says Barbara, smiling.
Barbara and Sean had been friends since the late 1980s, when they met during filming of Playing For Real, a TV play about Subbuteo.
Professionally, they collided at regular intervals, including starring in panto together. And when Barbara went on to find fame as evil Ella Cotter in Rab C Nesbitt, Sean was making regular appearances in the series playing Rab's Anglo cousin, Shug.
After Barbara's marriage crumbled, romance blossomed.
"When, Love Lies Bleeding, ended I realised I was missing Sean," said Barbara.
Since then they have been inseparable, in life and work. But how does it work out?
"Some couple don't like to work together, but we do," says Clydebank-born Barbara.
"I think it's because we get on so well. We're quite similar.
"One of my sons asked me what Sean and I talk about and I said 'Acting.' We do talk about other things but it always seems to come back to shows we loved or films we've seen.
"We've got a real fondness for the world. And we both see acting as a craft. We want to get better.
"Acting is about alchemy. It's about creating magic on a stage. That's what you strive for."
There's little doubt Barbara and Sean have the alchemy, and the talent, to make their latest theatre show a success.
Barbara's star has shone in TV shows like Hamish Macbeth and River City, and on stage with the Raindog theatre group and in Chitty Chitty Bang Bang, as the basque-wearing Baroness Bomburst of Vulgaria.
Sean has also had a sparkling career, from working at the National Theatre to TV dramas such as 2000 Acres Of Sky and more recently in theatre success Sins Of The Father.
The pair, Barbara reveals, also had a hand in creating Lark And Clark, which also stars River City favourite Frank Gallagher.
"We did some improvisation with Danny watching on," says Barbara. "And we came up with a few ideas, such as creating this theatre double act who meet up four years on.
"It's brilliantly written, but it's certainly quirky. And there is some black humour in it."
The actress admits it can be scary to go out in front of an audience with a new play.
"It can be frightening. Especially when its comedy. If you don't get that first laugh you get so deflated.
"I remember being really nervous going out on tour with the Nesbitt stage show. When I delivered my first line and got a huge laugh it came with such a tremendous relief."
What if it doesn't work?
"Thankfully, I've not worked on a turkey for such a long time," she says, grinning.
Barbara doesn't feel self-conscious performing with Sean, even though they are two people in a relationship – pretending to be two people who once had a previous relationship.
"Well, since you mention it, it does sound odd," she says. "I think what happens is that you both go into another zone when rehearsing these characters.
"What does help is that when we're at home we talk about acting and plays all the time. We take ideas apart and rebuild them. We can help each other."
Barbara's directing talents – she directed at the Edinburgh Festival last year – even helped Sean land a job.
"Sean was up for a part in Holby City, but he didn't have time to go down to London for the rehearsal.
"So what I did was film him reading the lines, direct his performance, and then send Holby the video.
"It worked. He got the job. But you see, he trusts me to give him notes and vice versa. We both like to do a really good job."
The work ethic was drilled into Barbara from an early age.
"I remember once working on a job and I saying to my mother I was rotten in it, she said."
"She came right back and said 'You've no right to be rotten!' And she was right of course.
"If you're passionate about this job you've got to give it your all."
There's double passion involved in this play.
"There is," says Barbara. "But that still doesn't mean we won't be nervous."
n Lark, Clark And The Puppet Handy, the Tron Theatre, June 14-18.