I N a compelling scene in The King's Speech the over-familiar therapist Lionel Logue asks the painfully correct Duke of York when he developed his stammer. The introverted duke, second son of the no-nonsense King George V and destined eventually to take the throne, is lost for words.

Logue then makes the point that no baby is born with a stammer. It would have been caused by some shocking event in infancy or childhood.

Quite what triggered the royal stutter remains a mystery, but we know more about the childhood stammer endured by writer David Seidler – an affliction which provided him, when penning The King's Speech, with a rare sense of identification with his subject.

"During the Second World War I was evacuated to America" he recalls. "Three ships left, two full of families and one full of Italian prisoners of war. One of the ships was sunk by German U-boats – the ship containing the Italians."

These stresses saw the onset of a stutter which caused him to undergo various therapies before the impediment retreated in his teenage years. The broadcast speeches of King George VI were, he remembers, an inspiration. "We'd listen on the radio and although he wasn't perfect he was capable of delivering rousing speeches capable of moving people to tears."

As we discovered in Tom Hooper's film, Logue had a hand in that.

The prince's impediment had a negative effect on his confidence. His brother King Edward VIII's abdication suddenly put him on an unwanted throne with a job description that included plenty of dreaded public speaking.

With a mixture of fresh air, exercises, tongue-twisters, song and shouting the salty language that originally earned The King's Speech an adult rating, Logue helped his client manage the problem.

Given that King George VI was revered by the generation who won the war and had an impeccable reputation as a family man, wasn't his part difficult to write?

"No, because in a sense I was writing about myself and my own stammer," says Seidler. "And actually he did have a terrible temper and swore like a sailor, although this perhaps was a result of his frustration with his stammer."

Initially Seidler's principal qualifications for writing The King's Speech were his solid track record as a screenwriter, his discovery that an elderly uncle had also been a client of Logue's and his understanding of the frustrations stammerers endure.

"It was difficult at first to learn much about Logue. He was a tiny blip on the radar in the story of the king's life, firstly because he was engaged in an unofficial capacity; secondly because at that time a stammer was referred to as a 'speech defect' and having a 'defect' made you a 'defective' person. You couldn't have a 'defective' person as king."

Then in the early 1980s Seidler made contact with Logue's son Valentine. "He very generously offered to show me his father's notebooks. In the context of this story, these were like the Dead Sea Scrolls, but he would only do this if I had the permission of the Queen Mother to continue with my project.

"She asked me not to pursue it in her lifetime. It turned out to be quite a long wait!"

Even after her death in 2002, Seidler did not return immediately to the project as he was struggling with throat cancer (happily, he recovered). After producing his first draft he then wrote the stage play, at the suggestion of his then wife, to sharpen the key relationship between therapist and prince. The screenplay was further enhanced by the discovery, even as shooting was about to begin, of more Logue papers.

Seidler was lucky with the casting for the film: Colin Firth, Geoffrey Rush, Helena Bonham Carter, etc. The writer is equally excited about his first stage cast, which has Charles Edwards as the King George VI, Jonathan Hyde as Logue, Emma Fielding as Queen Elizabeth and Joss Ackland as King George V .

"I'm delighted to have Charles for this wonderful part and Adrian Noble was my first choice for director," he says. "I saw his production of The Madness of George III in San Diego and his experience with the RSC shows that he is a master of histories."

The King's Speech, directed by Adrian Noble, runs at the Theatre Royal, Nottingham, from February 13 to 18 at 7.30pm plus Wed 2pm, Sat2.30pm. £11–£27.50 plus concs. Box office: 0115 989 5555 or www.trch.co.uk