Transcript for Diane Keaton on relationship with her brother, his struggles with mental illness
Reporter: To understand the significance of Diane Keaton's body of work, just check out her imdb page, from "Father of the bride", to "Something's got to give." We're just goofy when it comes to love. Reporter: All the way back to her oscar-winning role in "Annie Oh, well, la-di-da. Reporter: She's been la-di-da-ing our way for 50 years, combining quirkiness and charisma. Deemed Hollywood royalty by Hollywood royalty, honoring her with the lifetime achievement award in 2017. Can't imagine the '70s, '80s, '90s, 2000s without you. Love you, Diane. So does everybody. Reporter: But Diane isn't comfortable with all that adoration. Every generation responds to you in some way. How does that feel as you're walking down the street? It makes me feel like you're crazy. Because that's not the way it can be. I'm a weirdo, you know? Reporter: Did I mention she's hilarious? Her humor and fashion statements attracting 1.3 million followers on Instagram. I love Instagram. Reporter: But her latest project speaks straight from the heart. Brother and sister, a deeply-personal memoir about her relationship with her troubled baby brother Randy. What made you write this memoir? I became the family documentarian. I think everyone has a family situation, almost all families, in some way. And Randy was such an interesting character, because I think his mind was just unique and unusual. Reporter: Diane pored through the family's treasure trove. You say Randy was the single male relationship that was the most intimate. Yeah. He still is. Reporter: What did you mean by that? Look, I'm a person who's never married. And I think that Randy had the most significance, and it's being played out now more than ever, now that he's, you know, he's kind of infirm in a way that he can't really express himself too much anymore. I look back on Randy, I just think, wow, I wish I could have been a better sister. That's all I really do, I wish I could have been a better sister. Reporter: Their lives started out in the same bunk beds, southern California in the '50s, an idyllic suburban upbringing. We had a lot of happy times. We went to the beach. We were always at the beach. Always camping at the beach. And that was a very great time in our life. Reporter: Their childhood financially comfortable. And yet emotionally disjointed. Randy struggled. Had sort of a point of view of life that was always completely so sensitive, and he was so afraid. Reporter: When did you first get an inkling that something was wrong with Randy? I went off and I went to acting school, and I was like 19 or 20, something like that. He didn't have a direction. He didn't want to go to school. And he hated working for my and so it was sort of obvious that it was becoming unusual. I think mother and father, this was the dilemma of their life and the problem of their marriage, too. For dad, it was the son. He was so important. And for mom, it was protecting him. Reporter: A loner, an outcast, she describes her brother's life as being spent on the other side of normal. Battling severe alcoholism, unable to hold a job. Supported financially by her family. There wasn't a classic diagnosis in the sense that growing up in the '50s. It wasn't a time when people talked about mental health and No, no, none of that. Absolutely not. My career was starting to take off more, and I was gone almost all the time. I lived in New York, I was traveling around. So I didn't see Randy as often. So I wasn't really in the thick of it. Reporter: He drank a lot. Oh, yeah. Reporter: Did you ever directly tray to stop him? No. Reporter: Why? Because I just don't think that works. It wasn't like others hadn't it wasn't like he hadn't seen psychiatrists early on. Nothing, nothing made a difference. Reporter: As Randy's life floundered, Diane's career took off. You know how nave you sound? Why? Senators and presidents don't have men killed. Reporter: You talk in the book about audition for "The godfather". You thought it was a waste of Yes. Reporter: I was not the type of woman to play an upper class I don't understand what they were thinking. Reporter: And then a string of films with woody Allen, including "Annie hall." As her family's archivist she combed through her brother's letters and journal entries. Reporter: You made a movie called "Reds." Your brother writes you a there are times in "Reds", when I wanted to stop the projector so the movement wouldn't move so fast. Where did you learn to use your face so well? I think you ran across every emotion in the book, then threw the book away and made up some of your own. That was sweet of him. I don't know what to say. I'm sorry. Reporter: Randy's writing and his art work helped Diane reconstruct his tortured mind-set which shifted from disturbing imagery to lyrical poetry. I heard myself echoing out of the trees. One more sad voice. I heard my story reverberating in the air. Along with the other voices of failure. At the foot of the mountains. There I came to sleep. Reporter: What makes you emotional as you read his words? That it's sad to me, you know, that he had a difficult life. Reporter: Diane's life, in comparison, flourished off screen as well, adopting two children in her 50s. Dexter and duke. She was with them, filming "Something's got to give", when she found out Randy was dying and needed a liver transplant. In 2003, he received that new liver, a second chance, one she says he squandered as he kept drinking unapology etically. And yet when he gets that close to dying and gets a new liver. Oh, no, please don't even go there. Reporter: How upset were you? Well, yeah, we were all upset. He was out of control. He couldn't manage it at all. Reporter: Diane writes that Randy remained defiant after the transplant, telling her, I'm an alcoholic, and I don't care, I will never stop drinking. She writes the more Randy lived the more he became the boo radially character. He battles dementia, a change Diane says brings them closer. As time passes she works to build new traditions. He loves life. We have a ritual every Sunday where his caregiver and, you know, this therapist, we wheel him around, you know, to Venice, and we hit the yogurt store. And we all have yogurt, and we sit there and talk a bit. Reporter: A reminder for us all. That despite our failings, we can still cherish our loved ones, while there's still time. I adore him now more than ever. Reporter: I notice the to Isn't that beautiful? Reporter: It's beautiful. Look how beautiful he is.
This transcript has been automatically generated and may not be 100% accurate.