Chandor worked on that letter for weeks in 2010, revising the missive on train rides from Providence, R.I., into New York, where he was finishing "Margin Call." He wanted the language set before penning the rest of his short, 31-page script.

"I love survival movies, but I thought that you had to play the same narrative tricks," Chandor said, referring to techniques like voice-over and inanimate objects ("Cast Away"), flashbacks ("Touching the Void") or a video camera ("127 Hours"). "So it really became, can you really make a movie like this — which is just to show what you are doing to stay alive? If the guy isn't going to talk, how do you make it a swashbuckling adventure?"

That adventure begins as soon as the film does. Soon after Redford is heard reading his letter, the film cuts back to eight days earlier, when we hear the gentle lapping of water (the sound design, by Steve Boeddeker and Richard Hymns, is extraordinarily precise). For Our Man, the sound is ominous, signaling that there has been a breach in his sailboat.

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In the middle of nowhere in the Indian Ocean, the Virginia Jean has collided with a massive steel cargo container — the kind you'd see on the back of a semi trailer or atop a train car — that apparently has washed off a freighter. The resulting cut at the boat's waterline has partially flooded the sailing vessel; critically, the seawater has fried the radio and navigation equipment.

Our Man quickly needs to begin repairs to make his craft seaworthy. He's close to shipping lanes, but without a radio, GPS or radar, he can't communicate and see what the forecast holds, including a storm thundering over the horizon. He tries to teach himself celestial navigation.

"He doesn't have a lot of time to reflect," Redford said. "It's completely in and of the moment."

As much as "All Is Lost" is focused on the mechanics of staying alive at sea — how can you get food and water when your stores are fouled and nearly gone?— the film ultimately becomes an existential story of a man combating not just nature but himself.

Surprisingly, Chandor didn't have a particular actor in mind when he was writing the film — and certainly not Redford. But that all changed at the Sundance Film Festival.

When Chandor took "Margin Call" to the Utah gathering in January 2011, he was invited to a filmmakers' brunch hosted by Redford, founder of the festival. Chandor was seated far back in a room where Redford was speaking, but the public address system wasn't functioning properly. Chandor could see that Redford was talking, but couldn't hear a thing he said. A technician then connected a speaker right next to Chandor, and the actor's distinctive voice came booming into the director's ears.

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Chandor was immediately struck by an idea: What if you stripped a performer as well-known as Redford of something as integral to his acting as his voice? Too shy to ask Redford at Sundance about potential interest, Chandor waited until his brief script was completed. Just minutes into his first meeting with Chandor, Redford agreed to star in the film — startling the filmmaker with the speed of his decision.

Redford knew it would be a challenge: Unable to banter with other performers or slide by on his charisma and looks, the actor was forced to do things — acting on an almost elemental level — that he hadn't previously done. While Redford's character doesn't verbalize feelings like despair and hopelessness, he's clearly facing his own imminent mortality and knows that he may not have what it takes to get the job done.

"I think it had to do with trusting him," Redford said of accepting a part that relied on just doing, not saying. "Because before, whenever I would challenge myself about saying yes, it had to do with how much faith I had in that person — the director, the scriptwriter — to deliver the goods. And I had faith in J.C."

Chandor believes the journey of the character and the actor in the film's third act are eerily parallel. "He's forcing himself as an actor to do things he wasn't sure he could do," the director said. "He has admitted the physical sense — that he's going to push himself. Now, as an actor, he has to force himself to go deeper."

Initially, Redford and Chandor planned for a double to perform many of the film's more dangerous stunts, including being washed overboard and the Victoria Jean's capsizing. But once filming started, Redford volunteered to act in several action sequences that he and Chandor had assumed he would watch from the sidelines.

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"It was my ego," Redford said. "My ego jumped in and said, 'Hey, you can do this. You can do this. Let's see if you can do this. Let's see if you can jump off the boat and dive into the… Do it!'

"I was wet all the time. They hit me with water when I was dry; to do another take I'd have to go get into dry clothes again. It was not fun."

Redford said he asked for (and Chandor granted him) several scenes where his character was doing nothing but thinking — not repairing or swimming or struggling. Redford, who grew up in Santa Monica, said during those moments he thought of his youth sitting on the beach, imagining the enormity and beauty of the ocean, and how insignificant man is in comparison.

"I don't know how this will play out in the public domain. I have no ideas," Redford said of his worries about how "All Is Lost" will be received by general moviegoers after playing at festivals in Cannes, Telluride and Toronto this year.

"To be appreciated in a festival is one thing. When you go into common ground, it could be another," he said. "I mean, who knows? I just liked doing it."

john.horn@latimes.com