Her classic films include The African Queen and The Philadelphia Story and she still holds an Oscar record for four wins and 12 nominations, but Katharine Hepburn was haunted by a fear of performing. Despite her success as one of the industry’s foremost actresses, she saw the audience as her “natural enemy” and would repeatedly seek reassurance of her talent from those closest to her.

Her nephew, Mundy Hepburn, recalled that her anxiety was extreme when she came backstage after live performances: “Now and again she’d puke in the wastebasket, because she was so wired and scared and ‘Oh my God, what’ll they think? I’ve got to do a good job’ … Then she’d go on stage and be absolutely brilliant … She’d grab me by the shoulders. ‘Was I any good? Was I any good?’”

He is among participants in a new documentary on the star, which also draws on previously unheard audio tapes, unseen home videos and photographs, to which a British film-maker has been given access.

Titled Call Me Kate, it will be screened by Sky Arts on 29 May, 20 years after her death.

Lorna Tucker, its director, told the Observer that she was taken aback by the amount of previously unknown material, including hours of interviews recorded by Glenn Plaskin, a writer and Hepburn’s close friend, in which she had spoken of the great love of her life, Spencer Tracy, saying that she fell in love with him “almost immediately”.

Tucker said of the material: “Hardly any had been seen or used, only a tiny bit of the home videos … hanging out with friends, being silly, diving … There were reels still not digitalised, so it was incredible to be the first people to see it. Glenn Plaskin has never let anyone hear those audio tapes before.”

In the documentary, Mundy Hepburn also recalls the horror of his aunt discovering the dead body of her beloved older brother, Tom, who was aged just 16. She had gone to his room to fetch him for breakfast and found him hanging. She laid his cold body on the bed and was devastated.

The hardest part, she had said, was being told “to act as if he’d never lived” as that was how their father managed the family’s grief. Mundy Hepburn says in the documentary that the father, a doctor, loved his children, but he was so harsh that by today’s standards he was “probably abusive”.

In the audio tapes, Hepburn comments on her brother’s death: “I’ve come to the conclusion it was an accident.” She describes it as a hanging stunt that had gone wrong.

Tucker’s previous productions include a harrowing documentary on homelessness in which she revisited sites in which – as a teenage runaway – she had lived for 18 months some 25 years earlier. Titled Someone’s Daughter, Someone’s Son, it was narrated by Colin Firth. She said of Hepburn: “On paper, we’re completely different women, but the hardships she had to endure and the trauma she suffered as a child made me connect to her. I was also inspired by her resilience, to keep on walking when things didn’t go well.

“She was so insecure and she hated the way she looked, but she knew that she just had to play the game. She was an original icon and groundbreaking. She was, for example, one of the first actresses to produce her own films.”

Hepburn once described bargaining with two of Hollywood’s biggest players, Louis B Mayer and Joseph L Mankiewicz, over The Philadelphia Story screenplay: “Inside I was nervous, but held firm and told them what I wanted. $100,000 for the story and script, $100,000 for myself to star in it and $10,000 for my commission. The room went quiet. They tried to offer me $175,000, but I refused. I held my nerve.”

It worked. The film went on to be nominated for six Oscars. James Stewart won best actor and Donald Ogden Stewart won for his screenplay, adapted from Philip Barry’s original play.