Chris Lee
August 15, 2014 AT 04:00 AM EDT

Nobody, it’s fair to say, had a faster brain than Robin Williams. To see him on stage was to witness a kind of manic magical mayhem — an avalanche of improvisation, a fusillade of funny. ”Robin was a lightning storm of comic genius,” said Steven Spielberg. ”Our laughter was the thunder that sustained him.”

The laughter stopped on the morning of Aug. 11, when the Oscar-winning actor, 63, was discovered unresponsive at his Northern California residence in what the Marin County coroner’s office initially called ”suicide due to asphyxia.” Williams had been battling severe depression, according to his publicist, and his CBS sitcom The Crazy Ones had recently been torpedoed by mixed reviews and lackluster ratings. Long suspected of suffering from bipolar disorder — which Williams himself neither confirmed nor denied — the actor had survived decades of substance abuse, anxiety, rehab stays, and relapses.

News of his death sent a paroxysm of grief rippling through Hollywood. ”We have lost one of our most inspired and gifted comic minds, as well as one of this generation’s greatest actors,” Mrs. Doubtfire director Chris Columbus said in a statement. Williams was, in the words of Night at the Museum director Shawn Levy, a ”true and forever one of a kind.”

Williams exploded out of the stand-up world in 1978 as the star of the ABC series Mork & Mindy, playing an extraterrestrial on assignment, as it were, on Earth. The show made him, and his character, a national sensation, but success came with a cost. Throughout the late ’70s and early ’80s, Williams wrestled with cocaine addiction and alcohol abuse. ”Cocaine for me was a place to hide,” he told People magazine in 1988. ”Most people get hyper on coke. It slowed me down.” But he quit cold turkey in 1982, after his close friend John Belushi died of a drug overdose.

Clean and sober, Williams became a cultural lodestar, churning out hits that endeared him to fans (Good Morning, Vietnam, Aladdin, Mrs. Doubtfire) and dramatic films that impressed critics (The World According to Garp, Dead Poets Society). Over his career, his films raked in more than $5 billion worldwide, and his 1997 performance in Good Will Hunting earned him an Oscar for Best Supporting Actor.

But after abstaining from alcohol for more than two decades, Williams succumbed to temptation — a bottle of Jack Daniel’s — on a movie set in Alaska in 2003. His drinking escalated out of control again, and three years later he checked himself into a rehabilitation clinic. ”It’s just literally being afraid,” Williams told the U.K.’s Guardian newspaper in 2010 about his relapse. ”And you think, ‘Oh, this will ease the fear.’ And it doesn’t.” So what exactly was he afraid of? ”Everything. It’s just a general all-around arggghhh. It’s fearfulness and anxiety.”

A prolific workaholic who once made eight films in a two-year period (”He could not help but be funny all the time,” said Ben Stiller, who costarred in all of the Night at the Museum films with the actor), Williams attended weekly Alcoholics Anonymous meetings but admitted that his substance abuse and, moreover, the emotional roller coaster of sobriety had taken their toll on his marriages. He divorced twice, at an estimated cost of more than $20 million. ”Divorce is expensive,” he joked to Parade magazine last year. ”It’s ripping your heart out through your wallet.” Indeed, the expense of his second divorce had compelled him to return to television in 2013.

Despite the front-loaded humor of Williams’ roles, his work often betrayed a deep loneliness. Time and again, he portrayed men who feel isolated by temperament or circumstance: among them, Mrs. Doubtfire‘s divorced dad (who veils his true identity in drag to finagle visits with his kids), the obsessive picture-smuggling hermit Williams played in the 2002 psychological thriller One Hour Photo, and his lone-wolf crime writer that same year in director Christopher Nolan’s Insomnia. Even his genie in Aladdin is trapped alone in a bottle.

Last month, Williams reportedly checked himself back into rehab at Minnesota’s Hazelden Foundation to ”fine-tune and focus on his continued commitment” to sobriety, his spokeswoman said.

He had recently wrapped his final performance as Teddy Roosevelt in the third installment of the hit Night at the Museum franchise (due out Dec. 19) and had signed on to reprise his beloved role as Mrs. Doubtfire in a sequel to be directed by Columbus. But the work, it would seem, could not sustain him. The actor leaves behind his third wife, Susan Schneider, and three adult children: Zak, Zelda, and Cody.

”Robin was hands-down a comedy genius and one of the most talented performers I have ever worked with in television or film,” Mork & Mindy co-creator Garry Marshall said in a statement. ”To lose him so young is just a tragedy. He could make everybody happy, but himself.”

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