As Kat and Alfie in EastEnders, Shane Richie and Jessica Wallace have played out some far-fetched storylines, but one plot they still struggle with is very real and saw them fall out for years after private phone calls ended up splashed all over the tabloid press.

As rehearsals get under way for The Perfect Murder, which comes to the Bord Gáis Energy Theatre in Dublin next month, the pair sit backstage in the theatre musing on fame and its perils.

“The hard part, I thought, was the hacking business,” says Richie. “We were constantly being warned about hacking. People were saying the press could hear our calls. We were like, ‘F*** off’. Then stories started leaking. And because we were so close, I’d say, ‘I’m sure I told Jessie that’. And she’d say, ‘I can’t believe I told Shane that and now it’s in the papers.’ You end up turning on people.”

“We didn’t speak for five years because I thought he was telling stories about me,” says Wallace. “And he thought I was telling stories on him.”

“And we had no proof otherwise,” he says, shaking his head.

They are friends again and back playing husband and wife on the small screen. They will also be playing similar – yet very different – roles onstage in The Perfect Murder, directed by Olivier award-winner Ian Talbot and adapted by Shaun McKenna from a novel by Peter James.

On stage they will be Victor and Joan Smiley. That is, Smiley by name but not by nature. Their marriage is a miserable sham and Victor reckons there is only one way to get Joan out of his life. But things don't go according to plan.

“We’re a week into rehearsals,” Wallace says. “And it’s brilliant: a great cast, a great director. It’s a real dark comedy, very jumpy. It’s devilishly clever.”

Has their off-screen relationship – notwithstanding the five-year falling out – and their on-screen one made these roles easier? Wallace nods. “We’re best mates, so when we went into rehearsal we already had a head start. We know how each other works.”

The flipside is that EastEnders fans could struggle to divorce this new husband and wife from the pair's more enduring role. "That will happen," Wallace says. "It always happens."

She recalls playing a brothel keeper in a Lionel Bart musical last year. "It was a million miles away from Kat, and right at the end, when I'm kissing my husband, someone shouts out from the audience, 'Alfie won't like that.' And I was thinking, 'I just done all this and I'm still Kat.' But that is what happens when you're in a long-running drama."

The scars remain

The press mauling has left scars. "You're told when you come into EastEnders to be prepared to have your life under a microscope," Richie says. "When the youngsters come in now there is someone they have to sit with who says, 'Right, I need to know all your skeletons.' Because the f***ers, the parasites out there will find them. You either embrace it and you go, 'Okay I'm getting on this ride now, I'm gonna get the best seat in the Iveagh [restaurant], but at the same time they are gonna know who I shagged last week.' You can either ride it or lock yourself away from it."

Wallace's body language suggests she would rather be locked away from it. She arches her eyebrows when she hears that the Irish press are less voracious in their pursuit of kiss-and-tell tales. "I might move over here then," she says. "They don't do it in America either. It's only England. It's awful."

She is moving to Ireland, as it happens, for four months. The pair are filming an EastEnders spin-off later this year. The six-part series follows Kat and Alfie as they travel to Ireland to track down the son she never knew she had given birth to. How could she not know she had given birth? It's complicated.

"It is another universe," Richie says. "It is set in a little village. I can't tell you where. This village has a dark secret. I don't want to give too much away but it is dark, it's sexy, it's sinister, but full of charm and warmth. Imagine The Wicker Man meets Desperate Housewives with a bit of Broadchurch thrown in. It's not cliched Ireland like back in the day, when they did mad donkeys and leprechauns and people fighting in the streets."

Richie knows all about Irish stereotyping, having been “conceived in a Dublin carpark” and reared in London’s Irish community by parents from Coolock and Whitehall. His mother managed a shelter for victims of domestic violence, and his father ran Irish clubs in London.

That was in the 1970s, not an easy time to be Irish in London. “Every time a bomb went off in Kilburn or somewhere, the police were the first in to the Irish community,” he says. “Basically we were mirror-imaging what is going on today. ‘Are you Muslim? Oh you must be a f***ing terrorist then.’

“For me growing up I found it all very exciting. My mum and dad split for a while, and my mum ran a hostel for battered housewives. Then a bomb would go off and my dad would be taken away and I’d be spat at.

“I remember coming over to Mosney on my holidays when I was 13. Someone tried to set fire to a chalet and I was asked to leave when they found out I was English. In London I was this Irish boy, and when I came over here I was asked to leave because I was English.

“But I was the centre of attention, and there was something captivating about that, which I suppose has carried me through my career.”

Earlier this year the actor Charles Dance lamented the absence of roles "for people who come through the state education system". Wallace and Richie agree.

“I don’t think that’s down to the actors,” says Richie.

“It is down to the producers,” says Wallace. “I’d love to be at the RSA or the Royal Court. But if you’ve been playing a character in a soap, it’s really hard to get your foot in the door. It is bloody unfair.”

Richie recalls when Sam Mendes brought Charlie and the Chocolate Factory to the West End. Richie wanted to be Willy Wonka. "But [Mendes] did a piece, in the Guardian where he said no one from soaps or reality shows were going to appear in any of his shows. What was upsetting was not the fact that he mentioned soap stars but the fact that he said it in the same breath as reality shows. I was like, 'Oh for f***'s sake, what chance have we got?' "

Cheating on a Nolan sister

Richie recalls how negatively his arrival in Albert Square was received in some quarters back in 2002. “Some were saying, ‘We have another reason not to pay the licence fee now.’ ”

But viewers loved him, not least because he brought some humour to an often bleak show. "Then Alfie became popular and it was like, 'Oh, this is all he does'. In the '80s and '90s, when I was Mr Saturday Night, my head was above the parapet and they were like, 'We're gonna f***ing take you down, boy.' I'd already cheated on one of the Nolan sisters, so of course that's sacrilege right there," he says, referring to the end of his nine-year marriage to Coleen Nolan in 1999.

Like Wallace he has his scars, but he will not let the critics or the scandal-mongers grind him down or force him out. He still craves the attention and uses it for his own ends, much as he did as the child of Dublin parents growing up in London in the 1970s.

“I embrace fame,” he says. “It’s how much you want to give up. How much of your soul you want to sell.”

• The Perfect Murder is at Bord Gáis Energy Theatre, February 15th-20th