It’s been a rough year – Casey Kasem, Philip Seymour Hoffman, Lauren Bacall and Robin Williams…

He was a brilliant actor and comedian who could go from light to dark in the blink of an eye. It’s the kind of thing that you wish you could do in meetings or at networking parties.

Apparently the darkness took over, as Williams was found dead from an apparent suicide at noon on Monday. He was 63.

Williams touched every generation and demographic, making his entrance in a 1970s comic generation with Steve Martin, John Belushi, Dan Aykroyd and Billy Crystal.

Williams went on to become the alien Mork from Ork in his breakout hit ’70s sitcom. He was also the voice of the genie in “Aladdin” and a hyper disc jockey in “Good Morning Vietnam.” In “Mrs. Doubtfire,” he played a dad who dressed as a woman to see his kids, and in “Birdcage,” he played a gay man. He was an English teacher in “Dead Poets Society,” a scientist in “Awakenings” and a prisoner of war in “Jakob the Liar.”

But it was on a stage, in front of the lights, where Williams shined most brightly. The riffs, tangents and impersonations came rushing at the audience, a seemingly endless torrent.

“You get the feedback,” Williams said in a 2007 interview. “There’s an energy. It’s live theater. That’s why I think actors like that. You know, musicians need it, comedians definitely need it. It doesn’t matter what size and what club, whether it’s 30 people in the club or 2,000 in a hall or a theater. It’s live, it’s symbiotic, you need it.”

On a television talk show, hosts knew Williams barely needed to be wound up. Sometimes, he needed only an audience of one: Williams visited Christopher Reeve a week after the actor’s horseback riding accident, dressed in scrubs with a surgical mask and speaking in a Russian accent.

The roles became less prominent as he aged and a different generation took the spotlight. Last year, CBS cast him as the star of a sitcom, “The Crazy Ones,” in which Williams played the colorful elder statesman at a Chicago ad agency. The network had high hopes for the comedy, which also starred Sarah Michelle Gellar, but they quickly faded and the show was cancelled after one season.

The failure of the show, coupled with the failure of the independent film “The Angriest Man in Brooklyn” this year sent Williams into an emotional funk, sources told FOX411. But darkness was nothing new to him. It was no secret that the Oscar-winning actor had suffered for years from periodic bouts of substance abuse and depression — he made references to it himself in his comedy routines.

Williams joked about one fall off the wagon during a comedy tour, saying, “I went to rehab in wine country to keep my options open.”

Williams (center) with Matt Damon (l) and Ben Affleck (r)

During an interview with comedian Marc Maron in 2010, Williams seemingly dismissed what would be a career highlight for many actors. “People say you’re an Academy Award winner,” he said. “The Academy Award lasted about a week and then one week later, people went, ‘Hey Mork!”‘

Word that he had likely killed himself Monday at his San Francisco Bay Area home left friends in the Hollywood community grief-stricken.

“Robin was a lightning storm of comic genius and our laughter was the thunder that sustained him. He was a pal and I can’t believe he’s gone,” said director Steven Spielberg.

His good friend, the comedian Steve Martin, took to Twitter to express his condolences. “I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul,” he wrote.

Former “Tonight Show” host Jay Leno also looked back. “I saw him on stage the very first time he auditioned at the Improv and we have been friends ever since,” Leno said. “It’s a very sad day.”

Williams’ “Mork” co-star Pam Dawber was stunned.

“I am completely and totally devastated,” Dawber said in a statement. “What more can be said?”

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