(CNN) -- By tradition, Drama is represented by two masks: the happy face of the comic muse Thalia and her sad counterpart, the tragic muse Melpomene.
If Drama were Robin Williams, you'd need a million more.
He was a mask of howling laughter, a mask of wide-eyed innocence. A sneer. A frown. Even, at times, a blank.
If it seemed like we knew what went on behind the many masks, it was because Williams' quicksilver mind and boundless talent possessed enough energy to blow them right off his face. He WAS Mork. He WAS Adrian Cronauer of "Good Morning, Vietnam." He WAS Patch Adams, and "Aladdin's" genie, and Mrs. Doubtfire.
But he was also the restrained Garp in "The World According to Garp," and the creepy Seymour Parrish in "One Hour Photo," and the firm but compassionate Sean Maguire in "Good Will Hunting," the performance that won him an Oscar.
It was that side of Williams -- something raw and vulnerable, not something manic and boisterous -- that made you wonder: who was he when he took off the mask?
On Monday, Williams was found dead in his home in Tiburon, California. He was 63. Coroner investigators suspect "the death to be a suicide due to asphyxia," according to a statement from the Marin County, California, Sheriff's Office.
It's a cliché, of course, the clown who laughs on the outside while crying -- or dying -- on the inside. It's Pavarotti's Pagliacci and Charlie Chaplin's Little Tramp; Willy Wonka and Laurence Olivier's Archie Rice.
Show business history is filled with stories of comic kings who fought against depression and substance abuse, not always successfully. Jonathan Winters, Williams' hero, was institutionalized for a time. The effortless Dick Van Dyke once said he was "mostly drunk for 15 years." John Belushi and Chris Farley died of overdoses.
Mitch Hedberg, Freddie Prinze, Richard Jeni -- all funny men, all gone before their time.
There's no question that comedy can be a form of escape -- and recognition. Richard Pryor, one of the most brilliant comedians who ever strode on stage, was raised in a brothel, married multiple times, struggled with demons both societal and personal. He was ruthless -- especially on himself.
Yet he was scathingly, mercilessly funny. It was comedy that drew blood, comedy as catharsis.
Chris Farley, on the other hand, grew up in a close-knit, comfortable clan -- but also sometimes seemed to be running from something. His immersion in Matt Foley, the divorced motivational speaker who lived in a "van down by the river," was both hilarious and a little scary.
"We lose at least one great comic to suicide or ODs every year," tweeted comedian Michael Ian Black on Monday. "Our jobs are to communicate, but we seem to not know how to ask for help."
Williams, the son of an auto company executive and a model, was remarkably open about his own battles. He was a chubby, bullied child who found he could make others laugh. Later, on talk shows, he could be outrageously stream-of-consciousness, a bicycle careening down a hill without any brakes -- and, just as quickly, brutally honest, admitting to sadnesses and abuses and mistakes.
"His bearing is intensely Zen and almost mournful, and when he's not putting on voices he speaks in a low, tremulous baritone -- as if on the verge of tears -- that would work very well if he were delivering a funeral eulogy," wrote Decca Aitkenhead, who interviewed him for the UK Guardian in 2010.
Inevitably, he'd make light of the problem -- the great line, "Cocaine is God's way of telling you that you make too much money," is his -- but underneath there was a sense of a man wrestling with demons, a man wishing to put on another mask in an attempt to scare them away. It is a mask that perhaps only the most blameless of us have never tried on.
But if it's another cliché that comedians hide anger under all those laughs, Williams didn't fit the stereotype. With him, what was underneath was compassion.
It showed in his performances, notably the therapist in "Good Will Hunting." It showed in his participation in such causes as Comic Relief, which raised money for the homeless, and in his USO tours, entertaining troops in Kuwait, Afghanistan and Iraq.
He'd had open-heart surgery several years ago, and it made him reflect on mortality.
"It breaks through your barrier, you've literally cracked the armor. And you've got no choice, it literally breaks you open," he told the Guardian.
Sure, there were performances in which he seemed less than kind -- whether on screen, as a villain in "Law & Order: Special Victims Unit," or in his real life, with messy divorces that he described, with perhaps a touch of bitterness, as "ripping your heart out through your wallet."
Given all that, who wouldn't want to put on Thalia's mask, with laughter riding across the audience in waves, approval on demand, smiles all around? Robin Williams wore it well.
It's sad that, in the end, he exited with Melpomene.