When the Coen Brothers pulled off the coup of persuading Jeff Bridges to take the role of Rooster Cogburn in their new film, an elegant and quite moving western called True Grit, they got what in Hollywood terms represents a rare bargain: two actors for the price of one.

The first was Bridges, 61, a grizzled superstar who last year cemented his place in entertainment history when, at the fifth time of asking, he was summoned onstage at the Oscars to pick up a trophy that declared him to be the film industry's Best Actor of 2010.

The second was Loyd Catlett. At 56, he shares the flowing grey hair and extremely laid back demeanour for which Bridges is famed. And he has starred in almost as many hit films. But while most actors make a living from swaggering proudly into the limelight, he earns a crust by staying entirely in the shadows.

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For the past 40 years, Catlett has been Bridges's official stand-in. The men, who are very best of friends off-camera, have collaborated on dozens of live-action films, from The Big Lebowski to Crazy Heart and both versions of Tron. They look similar, sound identical (both say "groovy" a lot), and in many famous scenes are quite literally interchangeable.

It turns out that almost every Jeff Bridges film you've ever watched features two versions of Jeff. One is the accomplished veteran actor the world knows and loves. The other – often used in long-range shots and scenes involving camera angles where viewers cannot see the star's face clearly – is Catlett.

"If people ask, then I generally tell them that I sell bar napkins and swizzle sticks for a living, so that I don't have to answer a lot of questions," he said, over lunch near his San Diego home this week. "But if I have to, I tell them that I have three job titles: I'm an actor, I'm a stuntman, and I stand in for Jeff Bridges. If they want to find out more, I tell them to look me up on the IMDB."

Do that, and the extent of Catlett's sprawling CV will immediately become apparent. Several of the riding scenes in True Grit, for example, show him on galloping horses in the costume of Rooster Cogburn, using equestrian skills he honed as a child on a farm in Texas. He also performed Bridges impersonations for many of the action sequences in Iron Man, stepping into Obadiah Stane's heavy superhero outfit while the real Bridges watched proceedings from the safety of the wings.

For most of his adult life, Catlett's intriguing role has been kept hidden from the public. But Bridges lifted the lid on the existence of his alter ego at last year's Golden Globes, when he won Best Actor. After receiving the trophy, he leaned into the microphone and declared: "I want to thank my stand-in, Loyd Catlett. We've done over 50, 60 movies together, man. Thank you, Loyd!"

Catlett was watching on TV at home. "It shocked me when Jeff mentioned my name," he recalls. "I went 'Wow!"... It was very kind and quite touching. I felt extremely groovy about it. And of course I was very happy and proud and excited for him, too."

The rare name-check also helped to shed light on "stand-ins", a little-know community of Hollywood performers. Almost every big star will use one at some stage of movie production, but only a few will admit to it. And while a small number of actors employ long-term stand-ins, most will keep the same one for only a few films at a time.

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The lion's share of Catlett's work occurs when film-makers are preparing to roll cameras. "Jeff comes in and rehearses. I watch his moves and what he's going to do with the scene," explains Catlett. "Then I emulate his movements, exactly the way he does it, for next the couple of hours, so the director of photography can set up the cameras and lights."

His other duties come as a stuntman. In a scene that involves risk of injury to the star, an accomplished stand-in will often completely replace the "talent". As Catlett puts it: "I am expendable, he is not."

To pass muster as their alter egos, stand-ins must gain and lose weight and grow and shave hair to match star. For True Grit Bridges and Catlett were both required to cultivate extravagant beards. They compared facial hair by emailing each other photographs every few days.

That process only added to the close physical resemblance between the two men, which at times can cause confusion. "If we're shooting in a small town and people know Jeff's around, I'm often mistaken for him," says Catlett. "In restaurants people will say, 'There he is!' I tell them they're mistaken, but they think I'm just a film-star being shy. To make them go away, I use the old routine: telling 'em I sell bar napkins and swizzle sticks for a living."

Catlett first met Bridges when they were both cast in the 1971 film The Last Picture Show. He was a small-town boy from Wichita Falls in Texas whose ambitions to pursue a career as a professional rodeo rider had been quashed by a string of injuries. Bridges, 21 was a paid-up member of the Hollywood "brat pack", with superstar parents and a string of glamorous friends. The two young men clicked, and Catlett followed Bridges back to Los Angeles at the end of filming. The rest is history. In the 40 years they have worked together, Catlett says, the duo have grown progressively more alike, to the point at which their off-screen characters are now often interchangeable.

"There's been years that we've spent more time with each other than with our own families. So our mannerisms have adapted unconsciously, I think," he says. "We've got hold of each other in lots of ways. I ain't how I used to be and neither is he. We've grown more and more alike."

Today, Bridges divides his time between a house in Santa Barbara, California, and a ranch in Montana. Catlett keeps a home in San Diego and a ranch in Idaho. The men email most days and speak over the telephone a couple of times a week. If Bridges is considering a new role, he will send Catlett the script and ask whether he considers it to be "groovy".

When Bridges is preparing a role Catlett often collaborates on the research process. Sometimes that's a major job. In the 1980s, before they filmed Morning After, a Jane Fonda cop drama, Catlett was dispatched to Bakersfield in California with a video camera to interview retired policemen about their careers.

If life as the "other" Jeff Bridges has prevented him from pursuing an independent acting career, then Catlett isnt letting it get him down. "I'm not going to beat myself up because I'm not getting parts," he says. "I make a good living. It's a great life."

Only once has Catlett questioned the sanity of hisrole: in 2007, he got an email with photo of Bridges with a shaven head. "I went. 'Oh God, that's a first!' and rushed out to the hairdresser," he recalls. "A few hours later, I emailed him a picture of me with no hair. He called straight back – 'Wait, I was joking! They gave me a bald wig in make-up! I didn't think you'd actually shave your hair'."

The next day, Catlett got another email. "Jeff had gone out and had his head shaved, because he couldn't bear for me to have to wear a wig. He's a groovy friend, like that. He wouldn't leave me hanging."

Stars and their entourages

Mark Wahlberg

The American television series Entourage was inspired by Mark Wahlberg's real life gang of mates. These included his childhood friend Donnie "Donkey" Carroll, his former bodyguard Johnny "Drama" Alves and a rehabilitated drug addict called Eric "E" Weinstein, who has been Wahlberg's assistant for more than 18 years. Wahlberg once said of Weinstein: "I don't think I could go anywhere without him."

Elvis Presley

Known as "The Memphis Mafia", the King's gang of his childhood friends were charged with protecting and facilitating everyday aspects of Elvis's day-to-day life, as well as playing games such as "war" with him, that reportedly involved groups of people throwing fireworks at each other. If not on the payroll, Elvis would reward his crew with jewellery, cars and even houses.

Marilyn Monroe

During the latter part of the Hollywood bombshell's career, Paula Strasberg wasn't just her acting coach. She was also one of Monroe's closest confidantes, accompanying her to engagements all over the world. Strasberg met Monroe through her husband Lee (considered to be a founding father of "method acting"), and was said to have treated Monroe like family