The producers stipulate that only mothers can appear in Mum's The Word. 'I don't really know why,' says Bernie Nolan, 'because you don't have to be a murderer to play a murderer.'
But then she admits of her role as Robin: 'When you get into the part you find it does help. It involves doing the whole giving-birth thing at the beginning.'
Pause for effect, then: 'Simulated, obviously!'
Well, thank goodness for that, says this squeamish male.
Bernie had her only child, Erin, at the age of 38.
'That doesn't seem so old now but I was considered a geriatric mum at the time,' she smiles. 'They actually called me that in hospital.'
Where the surname Nolan is involved it is impossible not to talk family relationships.
Bernie was one of the Nolan Sisters, later known simply as the Nolans, who became the first millionaire all-girl band. Their signature tune was I'm In The Mood For Dancing and they had more than 20 gold, silver and platinum hits as well as touring with Frank Sinatra.
Bernie has let it be known that she does not wish to talk about the death of her mother at the end of last year but gracefully accepts condolences and speaks of the continuing closeness of the family.
In fact the all-female cast of five that takes Mum's The Word to the Kings Theatre, Southsea, also includes Bernie's sister, Maureen.
Bernie says: 'Apart from a few charity things, we last worked together 12 years ago in the band.
'The company phoned to ask me whether I'd be happy for Maureen to join the Mum's The Word cast, which was nice of them, but we are always looking out for each other for jobs.
'If I can't do something I'll suggest my sisters - nepotism is a wonderful thing!
'In any case I knew Maureen would be brilliant because she made her acting debut in the play about four years ago, which is how I knew how funny it was.
'Sarah White was in that tour, and I did Brookside with her, so it's lovely to be with both of them again now in Mum's The Word, and I'm sharing digs with Maureen.'
But then sisterly mischief emerges.
In a show where the five performers sit across the stage, Maureen's and Bernie's characters are placed next to each other - 'and we try to make each other laugh!'
Despite the sense of fun that bubbles out of Bernie, life has been far from easy for her.
She had a baby who died at birth before Erin was born, and later she had a miscarriage. So she can engage with the agonies of parenting as presented in Mum's The Word.
It was conceived in 1993 in Canada. Six women who had once been exciting, glamorous, professional actors were now exhausted, bedraggled, amateur mothers with 10 children under six between them.
They decided to get together to write a show about motherhood. They met regularly on Saturday mornings without their children. They talked, laughed, whined, complained and laughed some more.
Suddenly their first performance was just three weeks away. Although they had nothing on paper, they felt enormously better about their children, their skills as mothers and their lives in general - and they remembered each other's stories. So Mum's The Word was duly delivered on time.
Bernie describes her character as a Jack-the-lad single mum who is not very popular with men.
She says: 'I think Mum's The Word works because there is something in all the characters that strikes a chord with people.
'And every story in the show will find someone in every audience who can identify with it, because they are all true. Some people have said "That was over the top" but we can tell them it actually happened.
'The first half is a series of monologues and duologues which we act out. Then in the second half the five of us do a piece called Soapbox. We are all on stage the whole time and laugh at each others' stories.
'It's not Shakespeare but it's very well-written and very funny.'
She says it is 'primarily but not exclusively' for women.
'The other day we had a particularly large number of men in the audience and they loved it. The hoots and howls are amazing, and you can hear people saying to each other "That's you!" My own husband loved it.'
As a solo artist Bernie has enjoyed recording success and lead roles in some of theatre's best-known musicals - most famously Blood Brothers but also Annie and The Sound of Music.
Asked whether she now considers herself an actor who sings or a singer who acts, she reflects before giving her verdict.
'I do love acting and have been doing it for 12 years. I did Blood
Brothers from 1998 to 2000 and then spent two-and-a-half years as Diane Murray in Brookside and three as Sgt Shelagh Murphy in The Bill. Now there's Mum's The Word.'
But after seven years acting with no singing, she made an album and gave concerts with her own band - 'and I think I'll always be a singer,' she says.
Even that answer, however, turns out to be not quite definitive.
Bernie acknowledges that Blood Brothers, Willy Russell's dramatic musical, is the ideal combination for herself and her sisters because the key role of Mrs Johnstone is a big sing with numbers including the heartbreaking Tell Me It's Not True.
Linda Nolan played that part at the Kings last year, and she and Bernie are among four sisters who have played the central character.
That is an achievement that has earned them a place in the Guinness Book of Records - a recognition that both Linda and Bernie have pointed out with pride.
The other two? For the record, they are Maureen and Denise.
Mum's The Word is at the Kings Theatre in Southsea from April 24-26. Call 023 9282 8282.