The Oscar winner, who committed suicide on Monday at age 63, was candid about many of his struggles, which began soon after his rise to fame in 1978 on the ABC hit Mork and Mindy.
"I used to go to two or three clubs a night and then go to four or five people's houses, just keep going, going," Williams told PEOPLE in 1982. "The myth of living fast, dying young affected me, but I've come out on the other side of it."
And yet he would continue to struggle and experience deep bouts of sadness, which he hinted was a far more personal side of him than the one he displayed onstage.
"Do I perform sometimes in a manic style? Yes," Williams told NPR in 2006. "Am I manic all the time? No. Do I get sad? Oh yeah. Does it hit me hard? Oh yeah."
That same year, Williams told Diane Sawyer just after completing a two-month treatment for alcoholism that he had fallen back into addiction gradually.
"It's the same voice that ... you're standing at the 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 a possibility."
He underwent treatment again this past July for what his rep called a chance to "fine-tune and focus on his continued commitment" to his sobriety. His rep also revealed that he had been struggling with severe depression prior to his death.
In interviews with PEOPLE, several friends and former colleagues remember the actor's private battles even as he remained an unerring professional on set.
During the late '70s and early '80s heyday of Mork and Mindy, Williams was known as an affable, gifted breakout star – with a penchant for partying.
"We're in our 20s and it was Robin's birthday," recalls his costar Jay Thomas. "After the show shut down we had a birthday party in Robin's trailer and there's men, women, drugs, everything's happening. As the night wears on, he stays there. I don't think any of us were his close friends and I remember thinking, 'This is kind of odd. There's lots of people that wanted to be with him and he's with us.' He was kind of by himself with a bunch of strangers on his birthday. I even think he slept in the trailer that night and stayed on the lot."
But the actor was in a very different place some 15 years later, when he filmed the role that would win him an Oscar in Good Will Hunting.
"I didn't find him to be the manic person he sometimes was," says the film's co-producer, Chris Moore. "He was sober the whole time on our shoot – we had to make sure and take precautions. We could not have any real alcohol around. There are a couple of scenes where he is supposed to be drinking scotch in the movie and we had to make sure that it was not around. Even the prop bottles that weren't even in the shot couldn't be real alcohol. I am very positive he was sober."
Still, his demons lingered.
"Robin had a heart the size of his whole body," says Moore. "It's just sad that a guy like that can't find a place in this world. On the face of it he had money, fame, people around him who loved him. For real. And it didn't work. He just couldn't find peace."
Reporting by ELIZABETH LEONARD AND JANINE RAYFORD RUBENSTEIN
For more on Robin Williams's tragic death and his legacy as a comic genius, pick up the latest issue of PEOPLE, on newsstands Friday
If you or someone you know is considering suicide, please contact the National Suicide Prevention Lifeline at 1-800-273-TALK (8255).
The Williams family is asking well-wishers to send contributions to charities close to the actor's heart in lieu of flowers. Suggested organizations include St. Jude Children's Research Hospital, Challenged Athletes, USO, the Muhammad Ali Parkinson Center, the Christopher and Dana Reeve Foundation and Glide Memorial Church in San Francisco.