Diane Keaton wanted to "explore" the "mystery" of her mentally ill brother in a new book.
The 74-year-old actress decided to write her memoir 'Brother & Sister' to explore the "mystery" of Randy Hall, who was diagnosed with a variety of illnesses over the years, including bipolar and schizoid personality disorders but never anything definitive.
Speaking to People magazine, Diane said: "Over the years people did a lot of measuring of Randy's mental status, and it all came to naught. He was so hidden. I wanted to explore the mystery of him."
Diane was close to her brother - who suffers from dementia and now lives in a care home - when they were children, but thought some of his behaviours were "weird".
She recalled: "I wondered why he was always crying.
"Why was he afraid of the outdoors? That's weird!"
As the 'Book Club' star's career grew more successful, she saw less of her brother and regrets not being very attentive, as well as feeling concerned about the impact her fame had on Randy.
She said: "I think it's hard for anyone who has a sibling who's out there, throwing themselves into the world and getting recognition. I wouldn't be happy to have a sister like that. Nobody wants to be compared."
Diane was left upset when Randy confided in her about the dark fantasies he harboured at his lowest points, when he wife left him and he was drinking heavily, but she was never worried he'd act on them.
He wrote in a letter to the 'Annie Hall' star: "I became addicted to watching horror movies, hoping the films would have some gruesome murder of a woman...."
She said: "There was no indication he would, in anything he'd ever done. He didn't have that bone in his body. He wrote about them and did collages instead."