How We Met: Mary Portas & Katharine McMahon

Katharine McMahon, 51, is an historical novelist. Her works include The Alchemist's Daughter and the best-selling The Rose of Sebastopol, which was shortlisted for the Best Read at the British Book Awards last year. She lives in Watford with her husband and three children

Mary and I met almost 20 years ago, when she joined my local theatre group at the Abbey Theatre in St Albans for a production of Come Back to the Five and Dime, Jimmy Dean, Jimmy Dean. Initially, she seemed nothing like me. She was already on the board at Harvey Nichols, drove a fast car and was incredibly glamorous, sharp and funny. Meanwhile, there I was with two small children. My first novel had just been accepted but I was still struggling. I was teaching and had just become a magistrate, so it seemed a very straight existence by comparison.

It was a shared sense of humour that brought us together. She comes out with things that are in my head. I toe the line a bit, while she is more on the edge. We both see the absurd in things, but she points it out very vocally, whereas, for me, it comes out more in my writing.

We also share a close-knit, Catholic family background, which is significant. I often find people I hit it off with come from the same Catholic background I do. There is a self-consciousness and a sense of ritual and drama, which might explain our shared love of the theatre, and both Mary and I are very committed to family and old friends.

Mary had her first child around the same time I had my third and we were living near each other, so we spent a lot of time with the kids in parks and on holiday together. Now the kids are older and we live further apart, we like going to Mary's place in Dorset for long walks and pub lunches. People would be surprised at how well Mary relaxes – she is either full-on or completely chilled out.

Mary has had a huge influence on my life. I re-read Footsteps, a novel I wrote not long after I met her, and there she was in one of the characters. I quite often write about two women who contrast each other and that partly reflects our relationship.

Mary is very honest, which can be challenging, as when she tells me, "You can't go out in that skirt," for example. But there are ways in which I need to be challenged. As a writer, you take endless knocks and I have a very negative side, which she won't stand for at all. She has also been very good at drawing my ambition out of me and saying it is OK to want to do well, which is quite an un-British and un-womanly thing to admit.

Mary Portas, 46, is a retail consultant, journalist and TV presenter. As a visual merchandiser, Portas transformed London department store Harvey Nichols into a destination fashion outlet. She has since turned her expertise to shops throughout the UK for her Queen of Shops series on BBC2. She lives in west London with her partner and two children

When I met Kate I didn't immediately think, "This person is going to be my friend." It was 1990 and we were rehearsing a play together as part of a group at the Abbey Theatre in St Albans. I was working for Harvey Nichols and I'd turn up in my BMW, having driven from Knightsbridge, which I suppose was slightly bonkers – nobody else in fashion was doing that sort of thing. But the theatre was a fundamental part of my life and gave me a sense of security after I lost my parents quite young.

Kate was the beautiful leading-lady type and I was the character actress, which kind of sums us up; in A Midsummer Night's Dream, she was Titania and I was Puck. She was a bit daunting in some ways and seemed quite serious. And she had two kids, which was completely alien to me – at that point I didn't know anybody who had kids.

Then I found out she had the most wicked sense of humour and we just connected. We'd make each other cry with laughter until they had to stop rehearsals. I'd tease her when I brought along copies of Vogue and told that unfortunately I hadn't got Women's Weekly for her. And I remember throwing a huge tantrum during a run-through when she asked if I was the same age as her – I am, of course, much younger.

Kate was brilliant when I had my first child. She'd just had her third and I'd call and wail, "He hasn't slept all night! I can't bear it," and she'd reassure me by lying and saying that her son hadn't slept either. I was so shocked by motherhood. It's so tough, you really need someone like that.

I miss having her really near. When we do have time together it is very focused and special – little gems in a mad landscape. We do the theatre, dinners and holidays. Our friendship is like a comfortable pair of fabulous shoes that you can put on and know you are going to feel great.

When I have difficult times, Kate is who I talk to. She listens without judging and gives me advice from her heart. Anything to do with my children or the break-up of my marriage, it was her I spoke to.

I read her books and see myself in them – I'm always the naughty one. It's flattering, though – and I am the naughty one in our friendship. But I would never want Kate to be seen as "safe", because she has broken a lot of boundaries. She has juggled three children with being a Justice of the Peace, a visiting lecturer and a bestselling novelist. She builds her world larger and larger all the time; I love people who do everything they possibly can. n

'The Crimson Rooms', by Katharine McMahon (Weidenfeld & Nicolson, £12.99), is out now