When McQueen, 44, set out to make a film about blacks in America in bondage, bought and sold like commodities, he hadn't heard of Northup's book, nor of Northup's story.
"I just wanted to make a film about slavery . . . to illuminate it, to take it out of the pages of dusty history books and project it onscreen, to make the past present . . . .
"And I had this idea of a free man who is kidnapped and put into slavery - we follow him through his journey," says the director, a descendant of Grenadian blacks.
"I liked the idea, because the audience, everyone, becomes that person who has been kidnapped."
So McQueen went to work with screenwriter John Ridley on their historical fiction. Then McQueen's wife, the cultural critic Bianca Stigter, discovered Northup's book. Historical fact.
"As soon as I had it in my hands, I never let it go," said McQueen, in Toronto last month for the movie's debut at the Toronto International Film Festival. "Every page read like a script, the language, its vividness, everything.
"And I thought to myself, well, how come I had never heard of this book before? This is crazy. . . . It became my passion to make this book. . . . It should be required reading."
And McQueen's movie - with Chiwetel Ejiofor as Northup - should be required viewing. A harrowing tale, realized with the precision and passion that marked McQueen's Hunger (about IRA prisoner Bobby Sands) and Shame (about a New York sex addict), 12 Years a Slave takes an unblinking look at one man's terrible story - and by turn, a long and terrible chapter in U.S. history.
"My sufferings," Northup later wrote, "I can compare to nothing else than the burning agonies of hell!"
McQueen, Ejiofor, and Michael Fassbender, playing a tyrannical plantation owner - and all the rest of this impressive cast - make those sufferings palpable. 12 Years a Slave, which was to be screened Saturday night at the Philadelphia Film Festival, opens Friday in select area theaters. It is sure to figure prominently in the coming awards season.
And, like Lee Daniels' The Butler, it is a film that addresses the legacy of slavery from the perspective of those most deeply affected by it - blacks.
By last month, McQueen had yet to see The Butler, the story of an African American who served under eight presidents in the White House, a witness to sweeping changes in the civil rights movement from a unique vantage - inside 1600 Pennsylvania Ave.
But McQueen cites a "perfect storm" of events, and chronology, by way of explaining the current keen interest in these kinds of stories.
"The unfortunate death of Trayvon Martin, and the 150th anniversary of the abolition of slavery, the 50th anniversary of the march on Washington - and no small fact that there's a black president," he theorizes.
"If you don't know your past, you'll never know your future."
As steeped in history as 12 Years a Slave is - and the period details are meticulous - there's another aspect of the story that fascinated McQueen.
"I always thought this film was like a fairy tale," he explains, likening Northup's seduction, and abduction, by a pair of trickster slave traders, to what happens to Pinocchio when he encounters the conniving con men in Walt Disney's classic cartoon.
"I always looked at it as something akin to that, where the two guys come and seduce Pinocchio and lure him into the circus. I always thought of the Brothers Grimm and children's stories, very dark."
Very dark, indeed.