Earlier this year, the pair had got the call about potentially reviving the soap opera, which aired its final episode on 4 November 2003.
Johnston says: “I couldn’t persuade him to do it. He wouldn’t do it, would he?”
“Not at first, no,” agrees Tomlinson, who played Bobby between 1982 and 1988.
“And then I thought, because we’ve both [been granted] freedom of the city of Liverpool, I thought we’ve got a duty really to promote Liverpool, get more young actors and people into the business working.
“I’m delighted I decided to do it.”
While the storyline for the special episode was a close-kept secret, they knew it would be a crossover with Hollyoaks to mark the soap’s 30th anniversary.
Both were conceived by Grange Hill creator Sir Philip Redmond, with Brookside later paving the way for the creation of Hollyoaks – also filmed in Liverpool.