"He's a brilliant guy, you know?" he added. "Does he have flaws? Yes. He has admitted it."
Williams said he did his drinking on private time, not while he was working.
His latest film, "Man of the Year," is due out Oct. 13.
It's a provocative look at a comedian who is elected U.S. president because he tells the truth.
Williams says the film reflects what he sees as a lack of integrity and accountability in politics today.
"I look in my own party, to both parties, to say, where is the courage?" he said.
Williams said before he traveled for work again he would return home to San Francisco to spend time with family members, who encouraged him to get help and who keep him going by telling him they love him.
"That's the bottom line," he said. "That helps the most, and then the rest is up to me, and God. And then you got to keep working it every day."
Watch Part 2 of Williams' interview with Sawyer on Monday, Oct, 9 on "Good Morning America."