Robin Williams normally doesn't fail to make people laugh.
But he took on a serious issue -- his battle with alcohol abuse -- on "Good Morning America" today.
Watch part two of Robin Williams' interview, Oct. 9 on "Good Morning America."
The 55-year-old actor-comedian spoke to anchor Diane Sawyer, two months after he checked himself into rehab for alcoholism.
Williams had been sober for 20 years when he started drinking again. He decided to seek help in August, his publicist said.
Williams said that falling back into alcohol abuse was "very gradual."
"It's the same voice thought that … you're standing at a precipice and you look down, there's a voice and it's a little quiet voice that goes, 'Jump,'" Williams told Sawyer.
"The same voice that goes, 'Just one.' … And the idea of just one for someone who has no tolerance for it, that's not the possibility."
Williams struggled with alcoholism and cocaine abuse in the early 1980s but quit cold turkey after friend and fellow comedian John Belushi's fatal overdose in 1982.
After two decades of sobriety and success in his career and family life, what made Williams fall off the wagon?
"It's [addiction] -- not caused by anything, it's just there," Williams said. "It waits. It lays in wait for the time when you think, 'It's fine now, I'm OK.' Then, the next thing you know, it's not OK. Then you realize, 'Where am I? I didn't realize I was in Cleveland.'"
Williams said he had spent two months "club medicated" in Oregon's Hazelden Springbrook treatment center.
He said he was treated only for alcohol abuse, not other drugs.
"Just alcohol. That's enough really," he said.
Williams said he had spent a couple of years thinking he could handle his alcohol problem on his own.
"But you can't. That's the bottom line," he said. "You really think you can, then you realize, I need help, and that's the word."
Williams' wife of 17 years, Marsha, and three children, Zelda, Cody and Zach, provided added incentive for him to seek help.
"It's hard admitting it, then once you've done that, it's real easy," Williams told Sawyer.
"You think people don't notice. Then you find out later, 'We knew. … You went outside naked.' No, I didn't. But even the dog was like, 'What's wrong, boy?' Humiliation gives you humility."
Williams told "Access Hollywood" that Mel Gibson's DUI arrest was a "wake-up call" for him to get help.
Today, he joked with Sawyer.
"First of all, I wanted to thank Mel. He took a lot of focus from me," he said.
Williams said he had "if there but for the grace of God" moments in his bout with alcoholism.
"And you stop because when you start violating your standards quicker than you can lower them, then it's time to go away," he said.
After Gibson's anti-Semitic statements during his arrest were made public, many fans wondered whether that was the real Mel Gibson, or whether he should not be held responsible for what he said because he was drunk.
Williams said he understands "not being yourself" when drunk.
"That's another side of yourself that even you don't know," Williams said. "He [Gibson] said afterwards, he wasn't there, like Doctor Mr. Jack Daniels. There's another person who appears."
Williams also suggested some in Hollywood have used a double standard to judge Gibson.
"A lot of people said, 'Don't work with him,' and also a lot of people said if his next movie makes money, they'll work with him." Williams said.