Duncan broke into show business as a bodyguard, so it's a miracle that he didn't get stuck in the hulking character parts he played (generic bouncers and tough guys) in his early movies. Credit goes in part to Bruce Willis (Duncan played part of his crew in "Armageddon"), who recognized something more in the six-foot-five, 315-pound actor and recommended him to director Frank Darabont for the key role in "Green Mile." (Willis and Duncan would go on to make three more movies together.)
On paper, the "Green Mile" role of framed death-row inmate John Coffey shouldn't have worked. He's more an allegorical symbol than a person: His initials are J.C., he has miraculous healing powers, and he voluntarily submits to execution by the state as a way of doing penance for the sins of others. He's the kind of character critics call a "Magical Negro," meaning that he's a black person with special gifts who exists only to provide the white protagonist with guidance; he has no hopes, dreams, desires, or inner life of his own. A "Magical Negro" character is often the sign of lazy writing at best, or of patronizing cynicism at worst.
Yet Duncan managed to breathe life into John Coffey. He deftly played up the contradiction between Coffey's physically menacing presence and his lamb-gentle soul -- a dynamic that would prove key to many characters Duncan would eventually play. Yet his performance was subtle, with Duncan letting his eyes and rumbling baritone voice do most of the work while his body remained still. His John Coffey may not have had much of a life, but Duncan convinced viewers he had a soul. It's no wonder his turn earned him an Oscar nomination; the film wouldn't have worked without his performance, and it's impossible to imagine anyone else in the role.
Duncan went on to show a gift for comedy as a grinning-but-dangerous gangster opposite Willis in "The Whole Nine Yards." It was a gift he would go on to exploit in many more movies, especially kiddie movies and animated films ("Cats & Dogs," "Kung Fu Panda,"), where his deep barking tone was always instantly identifiable.
Not that Duncan couldn't deliver pure menace. He was suitably scary as a gorilla general in Tim Burton's "Planet of the Apes," and he made for a truly frightening villain as crime lord Wilson Fisk, a.k.a. the Kingpin, in "Daredevil." (There was some mild controversy over this bit of non-traditional casting, since Fisk is drawn as white in various Marvel Comics titles, but one look at Duncan in the part, and you can see why the filmmakers chose him.) And he was hissably evil as Manute, a corrupt cop in "Sin City" (another movie featuring Willis).
Still, playing a walking paradox as a giant softie was just too irresistible to him and to casting directors, which generally led to comedic roles like his standout turn opposite Will Ferrell in "Talladega Nights" as Ricky Bobby's tough-love pit crew boss. Some of their funniest scenes together were in outtakes, suggesting that Duncan was skilled with improvisation as well as scripted acting.
Duncan was just 54 when he died Monday, though he appeared ageless. He might well have continued along his same path for the rest of his career, switching with ease between comic roles, cartoon characters, and tough guys and villains. Or maybe he might have found another "Green Mile" eventually that would have called upon his seldom-used dramatic gifts. All we can know is that he leaves behind a legacy of moviegoing pleasure, a glowering face that dissolves into a surprising grin and infects the viewer with unexpected joy.