The secret that haunts the Hollywood titan on his deathbed and which he believes could derail the murder trial of his Poltergeist actress daughter – and leave him on the brink of suicide

The brutal murder of up-and-coming actress Dominique Dunne, star of the 1982 hit film Poltergeist, has traumatised her famous Hollywood family.

His father, Dominick Dunne, was already at rock bottom. Divorced and broke, his Hollywood success had faded and he had been forced to sell everything he owned, even his dog.

As the dreaded trial approached, Tina Brown, the new editor of Vanity Fair, encouraged Dunne to write a diary during the grueling proceedings. When it was finished, she published it. It caused a sensation, but Dunne left out a crucial part of the story.

Dunne's son, actor Griffin Dunne, has revealed in his new memoir that his father was haunted by his own secrets; ones he feared would derail the trial, shame his family and lead to his daughter's killer escaping justice.

In The Friday Afternoon Club, Griffin – best known for his roles in American Werewolf in London and, more recently, The Girls on the Bus – writes that it was only when his father was on his deathbed, dying of bladder cancer, that he discovered he had been having a 30-year affair with one of Dominique's best friends, several years his junior.

That moment on his deathbed helped Dunne gradually unravel the mystery of his father's life and he has now revealed for the first time the anguished diary entries that have never been made public.

Dunne's gripping tale also details how his close friend Carrie Fisher lost her virginity to him, and his endless encounters with stars like Frank Sinatra, Dennis Hopper and Harrison Ford, who at the time was the handyman for Dunne's aunt, author Joan Didion.

Dominique rose to fame thanks to the film Poltergeist and television shows like Hart to Hart and Fame. But her young life was cut short when she was strangled by her abusive ex John Thomas Sweeney on October 30, 1982.In the driveway of his West HollywoodHouse.

Artist Norman Carby had been a key witness in the murder trial, and his account of Dominique's abuse was considered a key moment. He faced brutal cross-examination from defense attorney Mike Adelson, who attempted to portray Dominique as a drunk and drug addict whose death was a tragic accident.

After his appearance, Griffin writes that Carby joined the family for a celebratory lunch. Mom told him that what he said on the stand had given her the only moment of hope since the trial began.

Neither Alex [Griffins brother] I also couldn't understand why Dad had chosen to skip that lunch, of all lunches, and not be with us to thank Norman for his testimony. But I noticed that he kept his distance from Norman as we all gathered in the hallway before entering the courtroom.

Sweeney was acquitted of second-degree murder in favor of voluntary manslaughter, much to the family's dismay. Dunne's account of the trial was published, as Tina Brown had promised, and he became a featured reporter for Vanity Fair, covering other high-profile criminal cases, including those of the Menendez brothers and O.J. Simpson.

But it wasn't until years later, in the spring of 2008, that Griffin began to unravel the mystery of his father's coldness toward Carby, when he traveled to Germany to see him while he was undergoing stem cell treatment.

After dropping off my bags, I knocked on his door, he writes. A guy a little younger than me opened the door, and it took me a moment to recognize him as Norman Carby, Dominique's close friend and the impressive witness at his murder trial.

Dad was in bed, grey-haired and desperately thin, when I went to hold him. You remember Norman, he said to me, at the trial.

As Griffin tried to piece together the puzzle – and figure out why this face from the past was standing before him – his father fell asleep, and Carby was left to fill in the gaps.

Dominique had acted as a matchmaker between the two men when Dunne was at his lowest point in life. They had fallen in love and supported each other for 30 years.

“We've been single for a long time,” Carby told Griffin, “but he never stopped taking care of me.

Griffin always wondered why the secret had been kept for so long. In the 1980s, there was no shame in being gay, especially when you worked in the entertainment industry.

The answer came four years after Dunnes died. Missing his father, he traveled to Austin, Texas, to the Briscoe Center for American History, where his papers are archived. There he found the notebook containing the notes Tina Brown had asked him to take years before.

Under the date of August 23, he had written:

Adelson [Sweeneys lawyer] is a dangerous and evil man who hates me as much as I hate him. I believe his hatred led him to hire a detective to investigate my background and my relationship with Norman was discovered. This detestable and cruel man will expose our relationship to discredit his testimony and my character.

He then came up with the aggressive line of questioning: “You've been having a secret affair with your close friend's father all these years? What kind of friend does that make you? What kind of father has an affair with the friend of one of his children? What's the age difference between you and Mr. Dunne, who I believe is considered an elderly person?”

In the diary he continued to swear that if his relationship with Carby was used to sway the verdict in Sweeneys' favour I would kill myself this time, I would really do it.

The sense of torture his father had endured – not only in facing his daughter's killer in court, but also in fearing the possibility that he might somehow be responsible for her acquittal – was unimaginable to Griffin.

Brief flashes of his face came to mind as I reread the passage in the Briscoe Center, he wrote. It was distorted and he was making little moaning noises I didn't remember hearing until now.

“Adelson turned out not to play that card on what must have been the longest day of my father's life.”

But he adds: “I would have done anything to be able to hug him and tell him how sorry I was that he had to go through all of this alone.”

Friday Afternoon Club by Griffin Dunne is published by Penguin Press