A year before the actor Chiwetel Ejiofor, born in England to a Nigerian immigrant family, began filming “12 Years a Slave,” he was in Savannah, Ga., making another movie when he signed up for a tour of the city. One stop left a deep impression on him: a 19th-century slave pen, used to hold newly arrived Africans before they were sent to auction, and had, he recalled, “a sequence of bolts high on the wall.”

“I said to the guide, who didn’t know anything about my background, I said to him, ‘What are those bolts in the wall?’ ” Mr. Ejiofor said recently. “And he said, ‘That’s the extra chaining for the Ibos.’ And I said, ‘I am Ibo.’ ” He paused, then continued: “That’s when you are aware that you were there as well. That it’s your blood, that someone with DNA close to yours was right in the middle of that situation. You recognize that you yourself were there. And that’s powerful.”

If “12 Years a Slave” is in the running for best picture and a slew of other Academy Awards, it is in large part because of Mr. Ejiofor’s powerful, disquieting portrayal of Solomon Northup, a free black man from upstate New York who, in 1841, was drugged and sold into slavery in Louisiana. It is a role to which Mr. Ejiofor, 36, obviously felt deeply connected, but one that, from start to finish, also taxed and drained him....