A lot of people believe 12 Years A Slave is the best film yet made about slavery in the United States. That doesn't make it easy to watch.

It also wasn't easy to edit.

"Editing is like a massive, 5,000-piece jigsaw puzzle," says director Steve McQueen. He's just arrived from Europe and is relaxing in a suite in a swanky West Hollywood hotel with the film's editor, Joe Walker.

Walker edited McQueen's previous feature films, Hunger and Shame. The two share a taste for experimentation. Walker was trained as a classical composer; McQueen comes from the art world. When they remember their first rough assemblage of footage for 12 Years A Slave, they use words like "horribly wrong" and "nightmarish."

"There was one scene that we were struggling with, a scene when Solomon [Northup, the film's main character] boards a steamship," remembers Walker. "He's being kidnapped, and he's transported south to be sold into slavery."

But editing that terrible, disorienting voyage to Louisiana proved a problem. It was taking too long, and was further complicated by the production's sizable investment to build an authentic steamboat. The 10-week edit was taking place in Amsterdam, where McQueen lives. So one night Walker, in a fit of frustration, went out and bought "some international prize-winning skunk."

"And I got really, really stoned," he admits sheepishly. Now, Walker isn't normally a pot smoker, he says; his usual drug of choice is peppermint tea. But this time he just let it rip.

"What was going on in my head was a combination of some of the images, sort of snapshots of the things we'd been working on all day," he says — images swirling around with the music of Dutch composer Louis Andriessen.

Walker started editing the steamboat sequence to an Andriessen piece called De Snelheid, and showed it to the director.

"It was amazing," McQueen recalls. "It abbreviated the scene, but you felt the journey because of the physicality of the music and [the sense that] you were coming closer to something. This inevitable dark ending, to be sold down the river."

The music eventually used in the scene, by Hans Zimmer, was inspired by Andriessen. Generally, though, Walker and McQueen tried to avoid music in favor of natural sounds: the ominous rumble of Louisiana thunder, or slaves laboring on a plantation.

"We used the hard percussive sounds of pressing; they're sowing seeds in a field," Walker says.

12 Years A Slave's editing has been described by critics as formal and classical, partly for its use of restrained, unbroken shots. They are partly a conscious refusal to look away from slavery's ongoing depravations, faced particularly in the film by a young woman named Patsey.

"There's this long sequence when Patsey is beaten, and that's an all-in-one shot," Walker says. "Something peculiar happens when you don't cut. In some way, you're sort of calling to your audience — that it's real."

At another point, the filmmakers wanted to illustrate both the physical and psychic traumas of slavery, together, in one wide-angle shot. As a punishment, Solomon Northup is strung by his neck from a tree for hours, so he's barely able to breathe.

"And we see him hanging, and doing this surreal dance with the tiptoes of his feet in the mud," Walker says. "And we aren't plying you with music, and we aren't making big comments — and we're not that close."

The physical distance allowed the filmmakers to not just show Solomon dangling from the tree in agony but also to see the other slaves, forced to carry on with their tasks all around him.

"To me, it always felt like it was sort of a great way of realizing the casual nightmare of it all," Walker adds quietly.

He says he didn't really have a hard time working on such scenes of brutality. What was hardest was when Northup's freedom is eventually restored.

"I was in tears in the cutting room when I saw that come in," he says. "There wasn't a single shot of that family reunion that wasn't heartbreaking."

Perhaps those big emotions are — partly-- why some people are afraid to see the film. It's made less than $50 million in the United States. But most moviegoers who've seen it are happy they did. 12 Years A Slave is the best-reviewed film of the year, according to the website Rotten Tomatoes. (It sent McQueen its own award, the Golden Tomato.)

And on March 2, 12 Years A Slave will be up for nine Oscars, including best picture, best director — and best film editing.

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