Sometimes the merger goes deeper because of the exceptional nature of the subject.
Such was the experience of the writers behind a new film, The Railway Man, a biopic of Eric Lomax. Talk to Frank Cottrell Boyce and Andy Paterson, and it is clear that getting to know the Scots signals officer who survived torture in a Japanese PoW camp, only to decades later forgive one of his tormentors, and it is clear that this is a job that will stay with them a lifetime.
The project began with Lomax's own bestselling memoir, The Railway Man, published in 1995.
"The book was so beautifully written," says Paterson, who first met Lomax 14 years ago. "It captured the voice of a man who told such a shocking and epic story, and you just wouldn't want to have done it unless you could feel you could do justice to the scale of it, physically, which meant it was going to be expensive and thus difficult to raise the money for, and to find a cinematic way to go beyond the book."
In the course of trying to do this, Cottrell Boyce and Paterson travelled many times to Lomax's home in Berwick-upon-Tweed, getting to know him and his wife Patti. Lomax could be an intensely private man about his wartime experiences, and, if pushed too far, the "shutters would come down", as his wife put it.
"There were lots of full and frank discussions," says Cottrell Boyce of his friendship with Lomax, "but there was also just the business of spending time with him. What was most impressive about Eric was that he had found a way to be happy."
The film divides Lomax's life into two, with Jeremy Irvine playing the young soldier sent to the PoW camp after the fall of Singapore, and Colin Firth the older man tormented by the horrors inflicted upon him and his fellow soldiers as the Japanese forced them to build the Thailand-Burma Railway, more accurately known as the Death Railway. Some 13,000 Allied PoWs died hacking a railway out of the jungle, together with 70,000-90,000 locals.
The writers had to find a way to tell this story, and to explain the traumas that came calling after the soldiers returned. "Nobody wanted to hear their story," says Cottrell Boyce. "We'd just won a war, we didn't want to hear about an Allied army that was defeated."
In Lomax's case, it was his wife, Patti (played in the film by Nicole Kidman) who took the first steps towards helping her husband find peace. Among those steps was contacting one of his tormentors, Nagase Takashi, who had written his own book.
The screenplay writers were determined to show that this was a story of a marriage as well as a man.
"Patti is a wonderful woman," says Paterson. "She refused to believe that her role in this story could amount to anything compared to what those men went through, which we understood, but at the same time, she represents the millions of families who have to cope with the wreckage of war."
That task, they both stress, is the same today for many soldiers' families. "One reason that Eric was so keen to get this film made is that the issues don't go away," says Cottrell Boyce. "People are still tortured, and soldiers still find it really hard to come home and they don't get the support that they need."
Lomax did not know what a big fish the film had landed in Firth till he saw The King's Speech star's picture on the front pages when casting was announced. Having the Oscar winner on board was crucial to pulling together the finance for a shoot that would take place in Scotland, Australia and Thailand.
"It was so hard to find anybody who could capture Eric," says Paterson, also a producer on the film. "It's one of the curses of getting to know people so well, you need to do justice to them. The only good thing about this taking 14 years was that Colin was old enough to play it, and kindly won an Oscar just at the moment we finally got the money together, which unlocked a lot of things."
It was Firth who recommended Jeremy Irvine, star of War Horse, to play the younger version of Lomax. Although Lomax was born and brought up in Scotland, neither actor has a Scots accent in the film. Paterson says they tried it in rehearsals but eventually decided against. "It always takes a part of an actor's range to be dealing with an accent." In the end, believes Cottrell Boyce, it was not necessary. "The cadences are there, the script is written very much the way Eric speaks. When I see those lines I can hear Eric saying them."
Another debate was how much could or should be shown of what Lomax endured. It was a difficult balance, says Paterson.
"When Patti saw the film she said: 'You've given half a percent of what Eric really went through.' We tried to do it in such a way that you could look away, but still know that there was a reason why these men were traumatised by what happened. We tried to show as little as we possibly could without dishonouring the experiences."
One of the most powerful scenes in the film is of Lomax being waterboarded. Irvine wanted the scene to look as convincing as possible. "We tried various special-effects ways of dealing with water," says Paterson. "He went a lot further than I wanted him to, put it that way."
Cottrell Boyce says it is not so much that you are trying to tone the violence down so that people can watch it. It is more that you are trying to make people love that character, and relate to him, so that when you see him being hurt, you feel it.
Paterson went to Thailand for the filming of the railway scenes. It was an intensely moving experience. The mosquito-ridden, 45 degree heat offered a microscopic glimpse of what it must have been like back then for men slaving for 16 hours a day on starvation rations. Unsurprisingly, not one person on the cast or crew complained about the conditions on the shoot.
While the cast and crew were in Thailand, they realised it was Lomax's birthday, so they put together a grand hello cum happy birthday message on Paterson's iPhone and despatched it to Berwick-upon-Tweed.
Lomax died in October, 2012, aged 93, without seeing the finished film. It has been left to Patti to support the film at premieres.
"I think she feels it has captured him," says Paterson. "It shows a tiny fraction of what he went through, but she accepts it shows enough to make you understand." He is delighted at the recognition she is receiving. "When she came onstage at Toronto there was a huge gasp as it was announced that the real Patti Lomax was there." A standing ovation duly followed.
"She deserves it," says Paterson.
The Railway Man opens nationwide on January 10. Special previews, Maltings Theatre and Cinema, Berwick-upon-Tweed, January 3-9.