Alicia Dennis
August 12, 2014 01:45 PM

Robin Williams could do it all: make an audience helpless with laughter, move them to tears, astound filmmakers with take after take of improvised comic genius.

“He was a guy who had the highest of highs while performing,” Bob Zmuda, the creator of the Comic Relief charity, tells PEOPLE. “But when he walked off that stage and went back to the dressing room alone, he could face the lowest of lows.”

RELATED: Hollywood Reacts to Robin Williams’s Death

As the world reels to process the reality of his death on Monday from an apparent suicide, those who knew Williams best share personal memories of his one-of-a-kind talent – and his tragic struggles with addiction and depression – in PEOPLE’s new cover story.

On the set of the CBS comedy The Crazy Ones, in what was to be his last television sitcom, Williams was a consummate pro – but “we knew he was struggling,” a set source says. In the middle of one scene, “his face changed,” the source recalls. “He looked so exhausted and profoundly, deeply sad. And then one minute later he pulled himself back together, and he nailed the scene.”

RELATED: Robin Williams Poses with Daughter Zelda in Last Instagram Post

Last month, Williams, 63, entered rehab to “fine-tune and focus on his continued commitment” to sobriety, his rep said at the time. It was a battle the brilliant and complex father of three fought for decades, first quitting alcohol and drugs cold turkey after the 1982 drug overdose death of John Belushi and later seeking treatment again in 2006. He was struggling with severe depression when he died.

PHOTOS: Remembering Robin Williams – His Life and Legacy

“The process of being a comic is a painful one,” says producer George Schlatter, who helped launch Williams’s TV career. But what he remembers best is Williams’s joy when he was in the spotlight. “For him, performing was sheer utopia,” Schlatter says. “He was a true original. He was more than a comedian, he was an adventure.”

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.

For much more on Williams’s amazing career and tragic death, including photos and friends’ personal memories from his childhood, movie sets and family life, pick up this week’s issue of PEOPLE, on newsstands Friday

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