After nearly 100 years, there’s still been only one Robert Mitchum

Robert Mitchum (pictured with Jane Greer in “Out of the Past”) would have turned 100 next month.RKO Pictures

The best Hollywood book title? That’s easy: Errol Flynn’s memoirs, “My Wicked, Wicked Ways.” The best subtitle? That’s even easier: “Baby, I Don’t Care.” It belongs to a Robert Mitchum biography. Of course it does. Lee Server wrote the book — and Mitchum had the attitude.

Mitchum, who would have celebrated his 100th birthday next month, says those words to Jane Greer in “Out of the Past” (1947). She’s just sworn that she hasn’t taken a penny of the 40 grand that somebody — gee, who could that be? — has lifted from gangster Kirk Douglas. Greer knows she’s lying. The viewer knows she’s lying. Mitchum does, too. Doesn’t matter. “Baby, I don’t care.”

In those three monosyllables preceded by a term of casual endearment you find the man’s gloriously inglorious persona: amiable yet wary, more than a bit roguish, thoroughly unillusioned.

Starting July 10, the Brattle will be offering a centennial tribute: double features most Mondays and Tuesdays through Aug. 29.

Just by the numbers, Mitchum’s was an amazing career, and numbers are the least of it. Don’t be fooled by his manner. He worked like a man who needed to moonlight to make ends meet. In 1943, the year Mitchum made his screen debut, he had no fewer than 20 credits. He eventually appeared in 115 feature films, and that’s not counting multiple TV movies and series episodes.

Mitchum’s last movie came out the year he died, 1997. Who else could claim to be directed by both Josef von Sternberg and Jim Jarmusch? He was Katharine Hepburn’s brother-in-law (“Undercurrent,” 1946”) and Bill Murray’s boss (“Scrooged,” 1988). He hosted “Saturday Night Live,” in 1987 — the post-show party must have been a gas — and played Philip Marlowe twice, in remakes of “Farewell, My Lovely” (1975) and “The Big Sleep” (1978). The former includes Sylvester Stallone in a bit part. Hollywood’s heaviest-lidded male, senior division, briefly shares the screen with Hollywood’s heaviest-lidded male, junior division. Believe in devolution.

Six Degrees of Robert Mitchum could go from “The Birth of a Nation” (1915), courtesy of Lillian Gish, in “The Night of the Hunter” (1955), to the “Pirates of the Caribbean” movies. That’s courtesy of Johnny Depp. In “Dead Man” (1996) Mitchum aims a shotgun at him and roars, “Where did you get that goddamn clown suit, Cleveland?”

Even without a scatter gun in his hands, Mitchum projects such a natural authority. He always seems taller than the 6 feet 1 inch he actually was. Rarely moving fast, he always moves exceptionally. “People think I have an interesting walk,” Mitchum said. “Hell, I’m just trying to hold my gut in.” The walk was part strut, part shakedown cruise: steps dainty, arms swinging from the biceps while the shoulders stay stiff. Has any star ever acted so much with his shoulders? Mitchum’s walk is as distinctive in its way as John Wayne’s. Having fun with this, Howard Hawks shot them moseying along on crutches at the end of “El Dorado” (1967).

Mitchum’s walk isn’t a swagger. So relaxed is the motion he could be dancing with himself. Painters have a term “negative space.” Mitchum had negative physicality. Yes, he made a lot of action movies. But where Flynn, say, or Burt Lancaster would prance and flex and swing from chandeliers, all Mitchum had to do was stand and light a cigarette.

As Mitchum aged, physical weariness took its place alongside world weariness. Somehow he managed to make looking tired look good. Instead of indicating a hangover, the bags conjured up what must have led to it. There’s a sense of real exhaustion to his title character in “The Friends of Eddie Coyle” (1973). Even so, Mitchum musters the energy while attending a Bruins game to let loose with the best exclamation in the history of onscreen Boston accents: “Num-bah faw, Bobby Aw!”

The source of Mitchum’s staying power isn’t hard to find. It’s also the great thing about him on screen — better even than the circumflex brows, the half-mast lids, the out-there chin, the even more out-there chest. He so obviously doesn’t give a damn, and Jane Greer’s duplicity is the least of it. “I stick my neck out for nobody,” Bogart snarls in “Casablanca.” Every moment on screen, Mitchum seems to be muttering, “What’s a neck?”

Stars are oblivious to the audience. That’s one of the things that makes them stars. Their job — their duty — isn’t just to be watched. It’s also not to take notice of being watched. What makes Mitchum such a wonder is his being oblivious, magnificently so, to everything : to stardom, to acting, to the whole bizarre business of being up there on a big screen.

This made him constitutionally incapable of overacting. Understatement and restraint are as much a part of what made Mitchum who he was as the dimple on his chin. James Agee famously wrote, “Robert Mitchum is so sleepily self-confident with the women that when he slopes into clinches you expect him to snore in their faces.” It wasn’t just with the women. If cool begins with the striking of a pose, then Mitchum was beyond cool. The only thing he ever bothered to strike was a match.

No star has been so insolent for so long. Mitchum’s insolence was as natural as a burp — and every bit as satisfying. It wasn’t a pose. Expelled from high school, he rode the rails and was arrested for vagrancy, briefly doing time on a Georgia chain gang. He worked the graveyard shift in a Lockheed Aircraft factory (where he was friends with Marilyn Monroe’s first husband — which might account for the weird dynamic between the stars in “River of No Return,” 1954).

In 1948, Mitchum was busted for marijuana possession. The conviction was later overturned, but not before he did a week in the Los Angeles County jail. “It was like Palm Springs,” he said, “but without the riff-raff.”

Studied insolence long ago became a given among male movie stars, and in no small part because of Mitchum’s example. He differed, though, from the Brandos and Deans and Nicholsons — and not forgetting Sean Penn, who bears enough of a resemblance that he could be Mitchum’s grandson. They make a fetish of being seen not to give a damn. Mitchum didn’t give a damn about not giving a damn. Baby, he really didn’t care.

Mitchum liked to say, “Look, I have two kinds of acting. One on a horse and one off a horse.” That’s not fair. His special talent was to make relaxation work for nearly every role he played. “The Grass Is Greener” (1960) is a drawing-room comedy, largely set in an English manor house. Mitchum is up against Cary Grant. You’d think he’d be lost: two left feet on an over-waxed dance floor. Instead, he and Grant are like trans-Atlantic versions of each other, all amused aplomb.

Mitchum practiced what might be called acting degree zero. Nowhere do its merits come through more clearly than in the contrast with that other underappreciated pillar of RKO in the postwar years, Robert Ryan. They costarred in “Crossfire” (1947) and “The Racket” (1951). Playing a murderous anti-Semite in the one, and a crime kingpin in the other, Ryan heaves and seethes. One look, and you know he’s the villain. As the murderous Rev. Harry Powell in “The Night of the Hunter,” he would have been a catastrophe. Instead, Mitchum’s heading-for-a-hammock style makes the character truly frightening. It’s not just that you can easily imagine Powell committing murder. No less easily you can imagine him getting away with it. “Night” is the greatest movie Mitchum made, and so much of its power has to do with the lazy assurance with which he lets the reverend’s evil ooze.

In a 1983 Rolling Stone interview, he was asked if there was art to his acting. “Yeah, sure, but you don’t go out there thinking like that. Where the hell would you be then? Look, take music. You can study it all you want, you can learn about time signatures, and you can know what legato means, and you can read it, and you can appreciate it, but if you haven’t got an ear, if you’re off-pitch, then that’s it. I’ve always had an ear.”

There was a musicality to Mitchum: the way he talked, the way he moved, even the way he listened. On screen, he’s hearing secret harmonies. The other actors are following the libretto. Mitchum is following the score. He usually knows the score, too, but that’s different.

Sometimes the harmonies weren’t so secret. That distinctive baritone, a bearskin rug capable of speech, can be heard affecting a Caribbean accent on his 1957 LP, “Calypso — Is Like So . . .” He co-wrote the rockabilly title song of “Thunder Road” (1958): “And there was moonshine, moonshine to quench the devil’s thirst/ The law they swore they’d get him, but the devil got him first.” Although Mitchum doesn’t sing it in the movie, his recording of the song spent 21 weeks on the charts. His 1967 LP, “That Man Robert Mitchum Sings,” includes a quite-creditable rendition of “Sunny.” It’s a lounge-lizard version, but a lounge where you’d definitely want to run up a tab.

The calypso album includes the lyric “Ah, Creole gal/ How you love your bacchanal!” She wasn’t the only one. There was always something slightly illicit about Mitchum. In “What a Way to Go!” (1964) he plays off that quality, making the acquaintance of Shirley MacLaine with the question “What are you doing after the orgy?” A cheerfully low-key loucheness, besides being no small part of Mitchum’s charm, has helped keep his performances from feeling dated. So much of screen acting from the mid ’30s until the late ’60s consisted of making the suppression of urges seem plausible. Suppression never appeared on Robert Mitchum’s to-do list. That’s why he was born to play Max Cady, the genuinely terrifying villain in the original “Cape Fear” (1962).

Cady could be the bastard son of Rev. Powell, with the sham morality thrown in the trash. If sharks smoked cigars and wore a straw hat, Cady could be Jaws — only Jaws is a lot nicer. “Night of the Hunter” is a great and enduring film. “Cape Fear” isn’t great, but thanks to Mitchum it’s enduring, all right.

Initially, he turned down the part. The producer sent flowers and a case of liquor. “I’ve smelled the flowers, and I’ve drunk the booze,” Mitchum admitted. “I guess I have to do the picture.” The response was pure Mitchum — and the performance is a purification of the Mitchum persona, the absolute nihilism that’s the logical conclusion of not caring.

The contrast with Mitchum’s costar, Gregory Peck, is both instructive and unintentionally hilarious. Peck’s Southern lawyer is like Atticus Finch on prissy pills. Calling Mitchum a “shocking degenerate,” Peck adds, “Why, I’ve seen the worst, the dregs — but you, you are the lowest. It makes me sick to breathe the same air.” Mitchum laughs and orders another drink. At the movie’s climax, Peck points a revolver at him. “Go ahead,” Mitchum says. “I just don’t give a damn.” Performance has become being.

“Other actors act. Mitchum is,” David Lean said after directing him in “Ryan’s Daughter” (1970). “Simply by being there, Mitchum can make almost any other actor look like a hole in the screen.” That is high praise. It’s even higher considering that the movie proved such a disaster — in large part because Mitchum was so miscast, as a meek Irish schoolteacher — that Lean didn’t direct another feature for 14 years.

It’s a tribute to Mitchum’s appeal that he was almost as miscast in the role that more people saw him in than any other. He was at least 25 years too old for Navy Captain Victor “Pug” Henry in the miniseries “The Winds of War” (1983) and “War and Remembrance” (1988). And a captain? He’d already played an admiral, “Bull” Halsey, no less, in “Midway” (1976). Not that the miscasting mattered. The ratings were fabulous. Baby, the TV audience didn’t care.