“What does he become? What kind of MONSTER?” In the original trailers for his 1963 Dr. Jekyll and Mr. Hyde comedy, “The Nutty Professor,” a riot of private neuroses running amok, Jerry Lewis asked the questions of any standard-issue horror movie of the time, with an atypically straight face.

We know now, of course. The chemistry professor Julius Kelp’s formula turns the schlemiel into a slicked-back compendium of Lewis’ own fears and desires, swinger Buddy Love. He was a little of Lewis’ former partner, Dean Martin; a little more of Frank Sinatra; a little more of his own, harsh, unpleasable father, the small-time Borscht Belt comic Danny Levitch; and, most of all, the other, darker half of Lewis himself.

It’s a stunning Janus of a performance, in what is generally (and I think correctly) considered his masterwork. Lewis died Sunday at 91, and as the tributes peppered social media, memories of one man’s lengthy, rangy resume of accomplishments, controversies and monstrous obsessions, the stuff of nightmares and dreams both, tumbled across the globe.

Lewis was lip-synching records for laughs when he was a kid, and singing “Brother, Can You Spare a Dime?” by the time he turned 5. When Lewis, all need and splayed legs and ear-splitting childlike lament, took the stage with Martin in Atlantic City, N.J., in 1946, at Skinny D’Amato’s 500 Café, a show business phenomenon combusted — for 10 years. The act lasted 10 very big years. When a list is made of the key pop cultural artifacts of wartime anxiety and the post-World War II atomic age, with its rhythms of fragmentation and a hint of the apocalypse, that list must include bebop, “Waiting for Godot,” Jackson Pollock and Dean Martin and Jerry Lewis.

In the day, accounts of their nightclub act spoke of its sheer, proto-punk disregard for the usual rules and regulations. It was shrill, a mess, a study in tension and release and when it worked, nothing worked better. Martin and Lewis conquered early television (on “The Colgate Comedy Hour,” among others), the movies (producer Hal Wallis kept them wearyingly busy), radio and live appearances. Then the atoms split, and for the better.

As a kid I discovered Jerry Lewis the great, by way of the older movies on television, right alongside Jerry Lewis the diminished movie star (“Hook, Line and Sinker” and “Which Way to the Front?” were the first two I saw in theaters). Like millions, I came to Lewis’ annual hosting of the Labor Day muscular dystrophy telethon, a fundraiser he did for 44 hotly debated years, not as a hate-watcher, exactly, but not without something other than pure adoration. Watching Jerry go off, lose his cool, brow-beat a guest or the audience: this was part of his Janus personality.

I interviewed him three times, and one of those times was what you’d call “dicey.” By the mid-1990s, Lewis had come back from a career low ebb in the late ’70s and early ’80s: the occasional bad film, strained and weirdly suspenseful nightclub appearances (I saw him at the Carlton Celebrity Room in Bloomington, Minn., the same venue Jose Feliciano played in the Coen brothers’ “Fargo”), bankruptcy, divorce, audience indifference.

Then Martin Scorsese put him in “The King of Comedy,” where all his ingrained, bone-deep authority and resentments and idiosyncratic charisma flowered as late-night talk show king Jerry Langford, stalked and kidnapped by freakishly devoted fans played by Robert De Niro and Sandra Bernhard.

He made his legit Broadway debut in a revival of the musical “Damn Yankees,” which he took on the road for hundreds of far-flung performances. I talked to him backstage before a matinee in New York, and then again a year later, in Los Angeles. Lewis made a big show of pulling out a copy of the first piece I’d written, angrily ticking off everything about it that ticked him off, though the harshest line in it was pretty benign, something about what it was like to talk to him and to try to keep track of the “intriguing cross-currents of various personalities — or, rather, glimpses of one enormously complicated one.”

“A lot of people in the press,” he told me, coolly, “think that if you’re multifaceted or very talented you have to be a s—t. I don’t know why they think that. My guess is, they make in a year what I make for two shows on a Saturday.”

We got the interview back on track talking about specifics. The cane routine, especially. This was the routine that the “Damn Yankees” revival (Lewis played the devil) made room for, in the Act 2 specialty number “Those Were the Good Old Days.” I remember watching the routine from the wings of the Pantages Theatre in Hollywood, as the chorus boy (man) with the fastest pitch tossed Lewis heavy, wooden, aggressively waxed canes at him from offstage, one after the other, as part of the number.

It was stunning. Lewis, then 70, was in terrific shape during the “Damn Yankees” tour and I remember second-acting the show more than once, just to watch Lewis get laughs on lines such as, “What is this, the cane mutiny?!” and his improbable klutzy-graceful misses and catches.

Like so many giants of his time, Lewis felt that he could and should do it all. As a solo filmmaker in his heyday, in the early ’60s, he created some of the most free-associative and frankly personal work being done within the Hollywood studio system. When he danced, in many of his movies, you saw the visual equivalent of everything he was doing verbally, with That Voice, the voice that he modified and perfected in the role of Julius Kelp. One of the great thrills of my interviewing life: hearing Lewis lapse, quietly but hilariously, into the Kelp voice as he explained the origin of that “Nutty Professor” character name, something he came up while writing the “Nutty Professor” script with his partner, Bill Richmond, aboard Lewis’ boat.

Off the coast of Point Loma, near San Diego, Lewis was floating in the kelp beds when it came to him. “My God,” he remembered thinking. “It’s Kelp!”

Those who never “got” Lewis, or don’t really know his work, might start with numbers such as the manic jitterbug from “Living It Up,” or his lounge lizard act from “The Nutty Professor.” On Jimmy Fallon’s show two years ago, one of his last and least controversial appearances, Lewis and Fallon riffed on the pantomime routine Lewis featured in “The Errand Boy,” scored by Count Basie, imitated by everyone from “Family Guy” on down. He was atomically gifted. He insulted women (they weren’t naturally funny, he persisted in saying), called the disabled “Jerry’s kids” and, worse, “God’s goofs.” His never-completed bid for serious filmmaking greatness, a concentration camp tragicomedy called “The Day the Clown Cried,” may now, with Lewis’ passing, see the light of day. I suppose I hope it does, since everybody’s been cackling over it for years (the script’s floating around online).

Then again, I sort of hope it doesn’t. The man’s gone now, and he gave too many people, civilians and professionals alike, too much pleasure across too many years for Jerry Lewis to become a mere joke. He was a huge, unlikely power surge and the biggest, strangest, most vital element in mid-20th century American comedy. In every medium.

Postscript: After our testy interview, the one in between the two less hostile ones, I ran into Lewis at Nate and Al’s deli in Beverly Hills, which we agreed was the best New York deli in LA or New York, even. He felt bad about losing his cool, he said, and wanted to pay for my lunch. I declined, he insisted, I declined, he insisted, the vein now going boinnngg in his neck, his voice getting a little louder. I declined. This was up by the front counter, by the unstaffed cash register.

The phone started ringing, and nobody answered it, and Lewis instinctively did what he knew he should do at that moment: He picked up the phone and started taking orders like a maniac in That Voice, the Julius Kelp voice, and the customers at the booths nearby cracked up, and I couldn’t quite believe it was happening.

He came up with something better than picking up a check.