It’s exactly a decade since Emilia Fox last trod the boards. After an unhappy experience in a savagely reviewed West End revival of Les Liaisons Dangereuses, the Silent Witness actress swore not to endure the baptism of fire of the London critics again.

“Why would you do that to yourself?” she asks reasonably.

“Although I didn’t come off too badly, I’d always said I’d only do theatre if the equation was right and when I read this play, and met (director) Peter DuBois, and heard the cast they’d put together, I thought, ‘Damn! The equation is right.’”

Although it’s fair to say she’s not looking forward to press night, Fox has thoroughly enjoyed the rehearsal process and passionately engaged with the arguments in Gina Gionfriddo’s Rapture, Blister, Burn, which she finds “heartbreaking and penetrating”.

The writer of hit Almeida play Becky Shaw, and the Kevin Spacey House of Cards, has penned a smart, funny piece about feminist theory and how women’s lives have changed – or not – since the 70s libbers marched for equality.

“Gina has cleverly put across the ideas of these great feminists to see whether they integrate into real life situations,” says Fox, who plays high-flying academic Catherine, newly returned home to tend her ailing mother Alice.

“Hopefully people will come, see bits of their life and it will produce a discussion – that’s what I felt when I read the play, I wanted to talk about it and find out where I fitted in with these theories.”

One discussion it prompted was with her actor dad Edward.

“Horror films and torture movies aren’t dad’s sphere,” she smiles.

“But I was talking to him about how roles have changed during his lifetime and he engaged with the discussions even though he’s an unlikely person to do so.”

Regret

Past 40 and single, Catherine is fearfully contemplating an impending lonely old age and regretting not having children.

Meanwhile her former best friend Gwen, who sacrificed career for an unsatisfactory marriage and motherhood is feeling unfulfilled.

“In Catherine, you see the vulnerability of a woman in her 40s without a family about to lose her mother who is the person who has loved her throughout her life.

“Gina is saying, there are great theories but the reality is maybe quite simple. You get to a certain point in life where the most extraordinary thing is the most ordinary thing – the thing you have to go beyond by searching for a career and your own independence.

“Whilst that’s fantastic, perhaps we are creatures who ultimately like to share our lives – for me personally sharing life is the ultimate happiness.”

Fox gave birth to daughter Rose three years ago and says motherhood utterly shifted her perspective.

“Our views and priorities change over our lives and certainly when you have children. I see things now I couldn’t have hoped to understand before. I have never known a greater love – who knew that love was inside you? – you love your family, I have found my way back to my family and they have been my greatest love, but the love you feel for your child – it’s like a lioness.”

Although there are “obvious comparisons” between Fox’s life and Catherine’s, they differ in the fictional character’s single-minded ambition.

Fox, who has split from both ex-husband Jared Harris and from Rose’s father Jeremy Gilley, delivered stand-out performances in Pride and Prejudice, Rebecca, Merlin and The Pianist and possesses the kind of luminous beauty the camera loves.

But she says “I slightly fell into my working life. Rather than setting about pursuing a career, I let things happen and I have enjoyed it so much.

“Now I have to juggle both a domestic and working life, and feel very fortunate in having both – but my primary focus and emotion is Rose. Family for me is way more important than anything else in my life.”

One of the play’s arguments is that feminism rightly encouraged women to have careers but was left them floundering with the impact on their home lives.

“It’s fantastic what feminism has done. We are striving for everything, but managing it all is complicated, and as Alice says, things were easier when there were clear boundaries.

“Catherine admits she wanted a family but didn’t do the stuff she had to do to get it. I have seen friends leave it much later to start a family and, for women, it can be too late. That’s heartbreaking. I so love being a mum. I would love to have had a brood of children but I didn’t start until much later because I was convinced I had to stand on my own two feet and make sure I paid my own way.”

She adds that the argument she finds “most heart-breaking and penetrating” is Catherine’s realisation that her choices might leave her alone.

“Intimacy is very hard to find and hard to hold on to and it is beautiful sharing your life with someone knowing that they love you. Maybe that’s a discovery to me, you get totally caught up on a treadmill of work and all the other stuff in life and sometimes don’t protect the relationship between two people.”

“How we negotiate all this fantastic equality is the question. Of course it can work, hundreds of couples do it, but it is a negotiation and you have to both make sacrifices.”

She saw it with her mother, the actress Joanna David.

“She made lots of career sacrifices to bring us children up. Family life was the most important thing to her and dad. But she’s having her time again – her career has come back in full force now we’ve grown up.

“Before I had children I would say, ‘If you want to go to work why don’t you just go?’ Now I have Rose I understand you don’t want to miss out on them growing up – the process of them separating from you is totally heart-breaking.”

Fox’s latest series as pathologist Dr Nikki Alexander is currently on air and she still enjoys the plots and challenge of Silent Witness which she films just minutes from home.

“I always said I would do it until I didn’t love doing it, and I still really enjoy it.

“They’ve been so amazing working around my pregnancy and post Rose, when I found it really hard going back to work and not being with her. I still find it difficult, hanging around the school asking, ‘Can I stay for the morning?’”

The only thing that annoys her is the oft-lobbed critical jibe that Nikki is ‘cold’.

“She’s not so dissimilar to Cathy. They’re both focused on their careers and have left their personal lives in the wings. But I have never played her cold – she’s totally led by her heart!

“I think it’s because she deals with a serious subject and is a strong woman and they think if you are strong you don’t have emotions, which is a lot of bollocks.

“I hope I am a strong woman and I am permanently overflowing with emotions.”

Rapture, Blister, Burn runs from today (January 16) to February 22 at Hampstead Theatre.