James Mangold, the director of Walk The Line, said he made the film because he was fascinated with singer Johnny Cash's life but felt his autobiographies left "huge holes in history".
By Helen Bushby
BBC News Entertainment reporter
"I told him I was a huge fan of his music and fascinated by his life, but essentially if what he wanted was someone who'd skip over the dark periods in his life then I didn't want to do the film," he told the BBC News website.
The film starts with Cash's troubled childhood in Arkansas, and sees him make it as a singer, embarking on tours with the likes of Elvis Presley and Jerry Lee Lewis.
James Mangold met Cash before his death in 2003
It also explores his first marriage, his subsequent relationship with country singer June Carter and his torment with drugs.
He said that Cash, who was "vulnerable, humble and shy", was "extremely gracious and sweet" about the prospect of a film being made about his life.
Mangold, who met Cash before he died in 2003, said he got the idea of making the film in 1995.
He had to approach Cash and his wife, June, for extra information about his early years because the two books the singer had written about his life were "largely useless".
"They were hagiographies, fond memories of growing up making music and in many places there were huge holes about the most controversial aspects of his life.
"I needed him and June and access to their family and the band and everyone else to fill in the gaps."
Joaquin Phoenix and Reese Witherspoon star in the film
Cash trusted him, he said, because "really good artists" do not want to "hear bull", and the director ensured he " got right to the point".
Despite the unflinching portrayal of Cash's drug problem, the singer was not concerned about how he was portrayed on screen.
"He was much more concerned about hurting others in his life," Mangold said.
"John was not someone who really subscribed to the ability to blame all your mistakes on other people so he was very concerned that his character own his mistakes in the movie."
But the director said he was "sad" that Cash's daughter Kathy from his first marriage said she was upset at the portrayal of her mother in the film as a "mad little psycho".
"We interviewed Kathy as well as her mother Vivian in the making of the movie, but it's very hard for kids growing up in a family seeing stuff like this dramatised in front of them," he said.
"All I can tell you is that for me it was important that John and June were happy about the script before we made it, and they knew the film we were making.
"It was also important to me that we as film makers felt we were telling the truth," he said.
The film's stars, Joaquin Phoenix and Reese Witherspoon, both got Cash's seal of approval.
Cash battled drug addictions during his long career
The film is largely a "love story", Mangold said, focusing on how their relationship developed over a number of years before they got married.
But he insisted it was "not a biopic", a term he said was a "Variety term, not a creative term".
"Ours is just a love story based on true events and co-mingles with the ascension of a great artist."
However it was far from easy for Phoenix and Witherspoon during filming.
"Singing live was their scariest experience - they were both terrified, neither are stage performers," he said.
"Half the audience could run to computers afterwards and reduce them to ashes. But they tried it and stayed steady."
He and his wife June died just four months apart
He was pleased with the result, and said Phoenix was "perfect" for the role of Cash, adding: "John's legacy was one of storytelling.
"He wasn't the best-looking guy in the room or the pearliest voiced in the room but he had the greatest sense of conviction.
"I think he was a very humble man, there was a quality of him of not presuming too much. He was very shy and music brought him out."