What if they came together, and it didn't work?
"I deliberately kept them apart until they met in the story, which is when the Somalis storm the bridge," says Greengrass, on the phone from a shutdown Washington, D.C., Thursday. "I think at first the young Somali actors were disappointed, because they wanted to meet the great Tom Hanks. But I wanted them to meet literally at the point of a gun.
"So, we rehearsed them separately for that scene, in separate parts of the ship, and then we shot it, and that is what you see. And I remember thinking, Well, this is the moment of truth . . . . And I just remember them coming through that door, barreling through it with absolute ferocity, and then Barkhad comes in and he says, 'Look at me! I'm the captain now!' which wasn't actually scripted. He just kind of found that moment, and it was electrifying. I could feel the hairs going up the back of my neck, and thinking this is going to work. It's going to work."
And it does.
Captain Phillips, which opens Friday, having premiered at the New York Film Festival late last month, is as taut, tough, and terrifying as movies get. Greengrass is no stranger to reality-based narratives - he started in British television, directing for the ITV current affairs program World in Action. He made the documentary-like Bloody Sunday, about the 1972 shootings of Northern Irish civil rights activists by British soldiers. His 2006 film, United 93, restaged the 9/11 hijacking of the United Airlines flight that crashed near Shanksville, Pa., killing all aboard.
When Sony Pictures offered Greengrass Captain Phillips - based on Phillips' 2010 book, A Captain's Duty: Somali Pirates, Navy SEALs, and Dangerous Days at Sea, and with Hanks already attached to star - it was pretty much a no-brainer.
"To be honest, it was an easy decision. It came down to three things," Greengrass explains.
"Firstly, it's just a great story, gripping, with twists and turns, layered and complex but also dramatic and exciting. So there was that aspect.
"Then there was wanting to work with Tom. We knew each other a little bit, but I've always wanted to make a film with him. And when you get a chance, you jump at it."
And, finally, there was a more personal reason.
"My father was at sea all his life, in the merchant marine. He's very elderly now, and I wanted to make a film on the ocean, for him . . . . I felt like I understood that world and what kind of guys those people are. It's a calling, the sea. A hardworking life.
"I'm going to show it to him next week. He'll be the toughest audience, I'm sure."
Greengrass, 58, has seen his biggest commercial success with the second and third installments of the Bourne trilogy, starring Matt Damon as the CIA operative with the memory issues. But even United 93, made for a modest $15 million with nobody like a star in its cast, grossed more than $76 million in its worldwide theatrical run, with millions more from video and television sales.
As with that film, Greengrass faced a dilemma with Captain Phillips: How to humanize the villains, the hijackers.
"You want the film to be layered and human, but obviously, I don't think there's any doubt in this film that these Somalis were bad people. On the contrary, I think the more you paint them as desperate young men with little to lose - I mean, you know, the most dangerous man in the world is the man with a gun and nothing to lose.
"But you've got to try to understand their psychology, what drives them . . . . And I think that's what makes the confrontation between Phillips and Muse so interesting. It goes to the heart of a divided world, and how do we respond to it."
But as realistic and rigorous as Captain Phillips is, it's still a movie, an "entertainment," and Greengrass has no illusions there.
"It's not journalism, it's not history," he notes. "But that's not to say that a movie can't tell truth. It can, and you have a responsibility to the known facts and you have a responsibility to the person whose story you are telling."
Since he entered into the postproduction phase of Captain Phillips, Greengrass had been considering what his next project might be. And he had been circling around Chicago Seven, another real-life story - about the arrest and trial of Abbie Hoffman, Jerry Rubin, Tom Hayden, and their fellow protesters at the 1968 Democratic National Convention.
Aaron Sorkin, Oscar-winner for The Social Network, had written the script, and Greengrass was this close.
"But in the end, for a bunch of reasons, it wasn't quite right for me," he reports. "But I think somebody's going to make a great film out of that."
"I'm just going to see what comes. See if that romantic comedy comes my way."
"That would be a change, now, wouldn't it?"