It wasn't just on screen that robin Williams broke the mold. So many directors left openings in the script knowing he would ad-lib. But it was also during interviews where he flew without a net if you... See More
It wasn't just on screen that robin Williams broke the mold. So many directors left openings in the script knowing he would ad-lib. But it was also during interviews where he flew without a net if you were lucky enough to keep up. Barbara Walters on the unscripted robin Williams, the friendship with Christopher reeve, the promise he kept. We were broke the other day, my thumbs are moving for my reason. What the Is that? A book. Reporter: An stage, he was a comic genius. In person it was no different. I'm running for mayor of new York. Reporter: To talk to robin Williams was a side split is unforgettable adventure. We met in 1989 after his huge success in "Good morning Vietnam" and dead poets society. His daughter Zelda was just six days old. No pictures, please. Do you want to introduce him to Zelda? She's only a week old but I feel this feeling. Do you have legos. Reporter: Our interview was in his Manhattan apartment. How do you like what we've done to your house? I feel you've done something beautiful. These are stained to give that lovely tech look. I like this. What you've done in this stuff, you can never have enough moving blankets in your home. Reporter: The crew tried but couldn't stifle laughter. You can never stifle found because eventually you'll be walking around your house going -- You're driving them crazy. This is called reality programming. The mafia interview. I never saw her. Reporter: His breakout role, the character Mork in "Mork and Mindy" was the perfect foil for his irreverent, ir repressable form of comedy. Since he played an alien, he didn't have to behave like regular folks, something robin rarely did anyway. I want to ask you all the dumb questions. Were you born like this? No. I guess it happened the first year of college. I store things. Basically, no, I wasn't performing all the time. I performed for my mother. Lenny Bruce used to talk about it, to love me. It was a way to relate with my mother. She was funny and the way to talk back to her was to be funny back. The first year of college I started performing in an improv group and I never stopped. A simple cough could send him into a riff. We have to do the Heimlich on you. I don't remember anything before 1945. What happened? I don't know. I was in the forest. I was picking nuts. I was delivering underwear to Gerber's and he was dancing like Betty Grabel. We woke up, we were in Poland. It wasn't anybody's fault. I think I'm warmed up now. Now we're back! Robin, did you ever just talk straight? Yeah. For how long? Hours at a time. Yeah? Yeah. I mean, I can, for a long time. I don't like to because it's more fun to play. Did you go into comedy because you couldn't make it as an actor? Yes. Is that true, that's what I read, that you couldn't get a job? You cry on this program I won't run any of it. You said you wouldn't cry. I couldn't make it as an actor. I remember doing hamlet and going, "To be" -- line. I found it difficult to be in tights. They can tell what religion you are. I can't believe it, Barbara. Oh, god. Forget that, robin. From the metropolitan opera house in New York City, Mr. Robin Williams. Reporter: On stage, robin threw away the rule book and refused to play it safe. He could even find humor in a disaster like chernobyl. We believe now that the accident has been taken care of. If you want to see the plant, I'll show you around. Reporter: And he could transform a simple stroll through the park into a master class of improvisation. All this money goes to the Barbara Walters foundation. An interview show is a terrible thing to waste. Reporter: That humor and humanity was a huge source of comfort to robin's friends, like his best friend, Christopher reeve. Reeve told me that when he was paralyzed in 1995, he was in his hospital bed, unsure if he wanted to live or die. And I turned to my side and there is robin Williams dressed as a doctor wearing one of those funny blue scrub hats. You recognized him? Of course. We've been friends for 22 years. With a Russian accent. Just like this doctor he played in the movie "Nine months". For the first time since I crashed I laughed as we always do with robin. I felt such joy seeing him. He had come all that way. It was one of the first indicators to me that life could be fun again. That sense of humor extended to his own health as well. He had heart surgery in 2 o009 and it was fodder for new material. They did an echo cardiogram and my heart was like beep beep beep. My cardiologist said that's not good. My Latin friends said, no, you can dance to that. You said at one point that even though they gave you a cow valve -- I ended up getting a cow valve which is cool because you can Standing up. That you would have preferred a horse. I was hoping to get a horse valve. I'll be able to run a nice quarter mile. Robin come out of gate number five. He's in the lead. Coming around the corner. But do you moo now with the cow valve? No. But I give a great quart of cream. It gave him a new perspective. The idea that we are mortal. That's a big wakeup. You appreciate little things like breathing. Like breathing? Yes. Breathing and walking and friends and family because you realize, damn, man, this is precious. Reporter: Most precious to him were his three children, Zachary, Zelda and Cody. How do you see yourself as an old man? Just hanging out with grandchildren and stuff, probably telling stories, remember when California was above water? If somebody said to you you have three wishes, you have so much right now you can have three wishes. First, many more wishes, okay, I'll go with that. See, we broke the rules already. For me I would like to see peace for the rest of all eternity, an infinite amount of peace and I'd like to see everything I say be funny for all time. Reporter: Even when he thought the cameras stopped rolling, he kept us laughing. It's free time, isn't it? It's play time. We've done the interview, all right, now it's time for the puppet show. The baby is in the back saying, let's go. Take a hike. Barbara is with us now. Whatever you would interview robin Williams the interview never really ended with the camera stopped rolling. That's what so wonderful. You planned all your questions and you knew what you were going to say and it was off the wall. I loved it. And he never thought the camera wasn't rolling. That's the best part, after the actual interview. You talked to him so many times over the years and did you ever really sense the true scope of his suffering? I didn't, but I'm not surprised because we now know that many cheomedians, people who are very funny, use the humor to hide the depression, as you saw earlier. But perhaps because he was so loved by his co-stars and fans across the country, that's why we're having the conversation tonight. To understand more about depression. When we come back here, robin
This transcript has been automatically generated and may not be 100% accurate.