Keeley Hawes plays Jen Grantham
As writer Anil Gupta explains: "Jen is her own worst enemy.
"She has a constant, nagging sense that things ought to be better. This makes it very difficult for her to be happy.
"Deep down she kind of knows it's her problem, but is unable to change herself, so she ends up blaming everyone else and usually Martin."
He continues: "Jen is superficially the least likeable character, but we always felt that people would be able to identify with her anger and frustration at life.
"We were thrilled when Keeley not only took the part but embraced that aspect of the character.
"Obviously having someone who looks like Keeley in the role helps the likeability!"
Keeley Hawes became a household name as Zoe Reynolds in the hit MI5 drama Spooks.
Other notable roles have included the period drama Our Mutual Friend and the controversial romp Tipping The Velvet.
Keeley starred in the moving ITV drama After Thomas and beat hundreds of hopefuls "to don her leg warmers" and play ambitious psychological profiler DI Alex Drake in Ashes To Ashes, the warmly received sequel to Life On Mars, a second series of which went into production this summer.
Keeley describes her role in Mutual Friends: "Jen is married to Martin; they're supposed to have been together for 15 years which I didn't like as I'm only just in my thirties!
"However, she's getting a bit twitchy and unsettled and she wants more from life. She would like another baby.
"They still love each other but she's just after a bit of excitement.
"Martin can't understand and says: 'We have holidays, we've got a new conservatory' but it's not enough for her; she's reaching that sort of stage in her life where she'd probably have wanted to have another child but they haven't and decides to go off and look for excitement."
She continues: "She's just a bit bored and she makes a mistake – and then she repeats the mistake, which is unforgivable I suppose.
"She sleeps with her best friend's husband – who also happens to be her husband's best friend.
"But it's not just a one night stand, as she does it again and this all becomes apparent in the opening of the drama at the funeral when she just blurts it out to Martin.
"She's a woman on the verge of a nervous breakdown and she's been seeing a therapist, so I think they've probably spoken about it and he's told her she has to tell the truth and tell Martin and just get it out.
"But, in fact, it's extremely selfish of her to impart this information at that specific moment – but then again, she tells him because there's no way they can move forward otherwise.
"She sort of blames him really. They go on a lovely journey together, but she has a long way to go to repair the damage she's done."
Keeley says she doesn't identify with what Jen does.
"I couldn't live with that guilt. I don't think she's a bad person and she and Martin do have a little boy together and she does want to make the marriage work.
"All of the characters are pretty selfish, but I hope that people will recognise that in themselves because people are a bit selfish aren't they – generally speaking if we're honest."
She laughs: "We have lots of brilliant flashbacks or fantasy sequences, which are a great idea – they just pop up in Martin's mind, his increasingly fevered imagination.
"There's me and Carl shagging in various locations and situations which Martin can't stop imagining and it gets wilder and wilder in his head and of course it starts to drive him mad!"
Jen has clearly been to see her therapist several times before she takes Martin along with her.
Keeley says: "Jolyon the therapist does, of course, come up with therapist speak; this is what's going to happen and this is how he'll react and you just have to stay calm.
"So she's prepared for a certain reaction. Instead, Martin's furious, as you would be, but she's two steps ahead because she's been seeing this guy and she persuades her husband to go to a session with her, much as he doesn't want to.
"She is doing it, though, with a view to making their relationship work and make it better and to make another go of it.
"She just assumes that he won't leave her – which he doesn't but I personally would never assume that. I think in real life you'd imagine someone storming out, but he doesn't."
She continues: "People do now go to therapy more often, which is a healthy thing to do if you want to make something work.
"Before there was a bit of a taboo with people thinking they had failed. There's no stigma attached to it anymore.
"But in fact the session is all about Jen's problems – how she can't continue to feel guilty for the rest of her life even though she is the one who is at fault."
She adds: "They stay together because he loves her, they've got a child and he comes round to the fact that perhaps he hasn't put enough effort into their relationship.
"But she then asks him not to tell anybody about this affair because the people it involves are close friends so it would destroy all the friendships.
"And when the secret is revealed via another route, she of course thinks it's Martin who let it slip."
Keeley really enjoyed getting back together with everyone to film the series almost a year after the pilot – now episode one – was shot.
She says: "We are all such friends, and you can't fake chemistry.
"Mutual Friends is about old friends and how we can fall in and out of love with them.
"Richard and Anil really understand women and the way they work.
"The series should appeal to fans of Cold Feet, Life Begins and Friends. It's a drama, but it's also very funny and has a terrific pace which brings out a lot of the comedy. I really enjoyed making it.”