Chaplin's comic brilliance and legendary womanizing made his life story a difficult one to tackle. No one was certain that Robert Downey Jr. could fill his shoes - not even Robert Downey Jr.

Robert Downey Jr. may be the only actor who routinely updates his own obituary. "This show could do in the 'ol career," he says during a break on the set of Chaplin. The 30 million dollar epic opens nationally this month with Downey in the title role. "I could be in a way over my head," he muses. "Way, way over. I mean, I could be known as the guy who destroyed Chaplin. Or from now on, known only as the guy who played Chaplin." He shrugs and shakes his head, then breaks into a hound-dog grin that creases the corners of his soft, putty mouth. "I guess the good news is I won't be remembered as the guy who played Pee-Wee Herman."

This quip is quintesential Downey - all ham and cheese and wry. It's his way of masking the incredible pressure that's on him to deliver a box office smash. A second-string Brat Packer who has left a vapor trail of mostly forgettable credits, Downey, at twenty-seven, is one clunker away from the Old Actors home. Hollywood was caught off-guard when he snagged the plum part of Chaplin over a wish-list of more bankable, more experienced actors. Just about every big name in town had lined up to play the transcendent Chaplin. Billy Crystal literally campaigned for it; Robin Williams, Matthew Broderick, Emilio Estevez - even Peter Riegert - took a shot.

With Downey, skeptics argued, they were entrusting the legacy of one of the most beloved screen figures of all time to a bit player with the range of, say, Gig Young. Even Universal Pictures, which initially owned the project, refused to green-light the film with Downey in the lead. "Thirty million big ones riding on this kisser!" Downey says, with an impish gleam. "Are we in trouble, or what?" Perhaps. But even if Downey doesn't deliver, the audience will get their money's worth visually. To simulate Chaplin's arrival in prefreeway Hollywood, the production tracked through miles of pristine orange groves in and around Santa Paula, a cozy farm community northwest of L.A. Nearby, in the town of Fillmore, stands a full-scale replica of Mack Sennett's Keystone Studio, where Chaplin got his cinematic start. Beyond that stands a life-size replica of the Paramount soundstage where The Gold Rush was shot.

Today they are shooting a scene at a local mansion that is serving as Pickfair - Douglas Fairbanks Sr. and Mary Pickford's legendary home. Dressed in a natty tweed sportscoat, Downey strolls across the lawn, in character, a paisley scarf knotted with panache at his neck. Kevin Kline, who portrays Fairbanks, twists playfully on an old-fashioned tree swing, grinning as his friend approaches for a rendezvous and early-morning cocktails. Downey doesn't look much like Chaplin; his body is huskier and more striking than the Little Tramp's. And yet, as he approaches the camera, Downey's face seems to transform. That unctuous Tony Curtis look of his, with its shameless smirk, takes on a certain stately vintage. He uses charm to ward off the impulse to caricature.

"Robert has exploited this opportunity to become a real actor," says director Sir Richard Attenborough (Gandhi). "He always understood the method, but perhaps lacked the right discipline. This time, he threw himself into the role with a degree of abandon that I still find astonishing." Diana Hawkins, Attenborough's longtime partner, admits that Downey was not the first name that came to mind when they were casting Chaplin. "We were looking for someone, um, different," she says, invoking an oft-used euphemism for British. "We were very aware that if we cast an American, the English would say, 'Hollywood's hijacked Chaplin.' " But then Downey showed up and improvised a scene they hadn't asked for - one of Chaplin as an older man. "Once we saw that test, there was never a thought given to casting somebody else."

The ambiguous undertones in their support are understandable. Downey's recent roles in Soapdish (1991), Air America (1990) and Chances Are (1989) were meaty, but for the most part, secondary ones. While critics contend that he has given some truly wonderful performances in some truly horrible films (like 1987's The Pick-Up Artist and Less Than Zero), in Chaplin, Downey has to hold his own against first-rate actors like Kline and Anthony Hopkins. If that doesn't flatten him, a reality-check comes when he shares the frame with Chaplin's daughter, Geraldine, who plays her own grandmother, Hannah.

There's certainly no shortage of beautiful young actresses on hand to bail Downey out, in case things start to drag. Charlie's angels include Penelope Ann Milier (Awakenings, Kindergarten Cop) as Edna Purviance; Milla Jovovich (Kuffs) as Chaplin's first wife, Mildred Harris; Diane Lane (The Cotton Club) as third wife Paulette Goddard; Nancy Travis (Three Men and a Baby) as aspiring actress Joan Berry; Moira Kelly (Billy Bathgate) as Chaplin's first love, Hetty Kelly, and his last love, Oona O'Neill; Marisa Tomei (My Cousin Vinny) as silent screen star Mabel Normand; and Maria Pitillo (White Palace) as "America's Sweetheart," Mary Pickford. "You could say that I've got my work cut out for me," Downey says, only partly in jest.

On the set, Downey is outwardly jaunty and unburdened. Nevertheless, it seems he has accepted the corresponding responsibilities that most leading men inherit as they clamber up the A-list. "Here, check these out," he says, pointing to tiny crow's feet at the corners of both eyes. "I didn't have them when we began [filming]. You know what everybody says: 'They give you character.'" He flashes that don't bulls - me grin. "But, to me, they serve as a reminder of what I've done on this show."

"What he's done has been nothing short of a remarkable stretch. Since the movie covers Chaplin's entire life - from his first stage performance at age five to his return to Hollywood at eighty-three - it required not only a crash course in Chaplin's screen 'oeuvre' but an in-depth character study. For the latter, Downey was given full run of the Chaplin archives at the Museum of the Moving Image in London. "They let me try on his Great Dictator outfit, as weIl as his shoes," recalls Downey. "It was like being backstage in Charlie Chaplin's world. Then, a week before we started shooting, I tried on his City Lights jacket. Being an American, I checked the pockets and found a cigar stub - probably left over from the scene where the millionaire has kicked him out and he gets a cigar off the street. I don't know, I guess the British never thought to check the pockets." Downey's most vivid memory of Chaplin is seeing him pick up a lifetime achievement Oscar at the Academy Awards in 1972. "I remember looking at him and thinking, I'll bet there's one incredible story lurking behind that face."

There is. Chaplin's downfall was women; he liked them often and young, beginning with a fifteen-year-old music-hall dancer named Hetty Kelly in 1908. Within a few years, Chaplin married actress Mildred Harris, an old woman by his standards - she was sixteen - when she claimed she was pregnant with his first child. Lita Grey, his second wife, was twelve when she met Chaplin, but they waited until she turned sixteen to tie the knot. Third wife Paulette Goddard... well, she's played by Cotton Club siren Diane Lane, which just about says it all. A scandalous fourth marriage to eighteen-year-old Oona O'Neill, as weil as a paternity suit by Joan Berry and a feud with the FBI's J. Edgar Hoover, precipitated his exile from America until nearly the end of his life. "I think Chaplin had about as tragic a life as God would allow someone," says Downey. "But I'm almost sure that it provided some of the most raw and best material for his movies."

The irony is not lost on Downey, whose own face once wore the barbed-wire stare of too-much-too-soon. Caught in Hollywood's spider web of attrition, he grappled with the proverbial vices: alcohol, drugs, fast cars and a soaring ego. Along the way, a seven-year relationship with actress Sarah Jessica Parker was thrown aside. "I was an impossible human being," he says, dropping his voice to an embarrassed whisper. The script for Downey's life was headed for a tragic, albeit all-too-familiar ending. He had developed a reputation for being trouble on and off the screen. But somehow -- inexplicably, he says -- the wildness took a constructive turn. "I confronted the dishonesty in my life and realized the jig was up," he says with startling frankness. "I had to do an about-face in growing up and acting." Chaplin is a major step in that direction. "It'll be tough to go back to making superfluous little comedies or trivial pictures," he says, grabbing his hat. He begins to waddle off with a Chaplinesque lilt, then stops to reconsider. "At least, until I'm broke."