Before Casey Affleck started filming “Out of the Furnace” last year, he felt, professionally speaking, as if he’d lost his way. Five years earlier, in 2007, he had two breakout roles: “Gone Baby Gone,” directed by his older brother Ben, and “The Assassination of Jesse James By the Coward Robert Ford,” in which he played the coward, Robert Ford, and for which he received an Oscar nomination for best supporting actor. In the years following, though, he had starred in only two live-action films (and provided a voice for the cartoon “ParaNorman”). His most recent and notorious creative project was in 2010, when he poured his heart, brain and wallet into “I’m Still Here,” a passion project he made with his brother-in-law and close friend, Joaquin Phoenix. The hoax-documentary purported to chronicle Phoenix’s real-life personal implosion, as he supposedly made the switch from acting to a rap career. The film not only met with many terrible reviews but actually seemed to enrage the media. Dana Stevens of Slate had a not untypical critical reaction: “The worst thing about ‘I’m Still Here’ is the fact that it exists.”

So when Affleck started shooting “Out of the Furnace,” an ambitious film about two brothers in steel-town Pennsylvania, he was looking to regain something. During a scene, sitting across from his co-star Christian Bale, he realized what that thing was. “What I saw Christian doing was, I felt like, some of the best acting that I’ve ever seen,” Affleck says. “You know when you sort of forget what you like about movies, and then you see a movie again that reminds you of why you fell in love with them in the first place? That was the experience.” After his first day of shooting, Affleck called Matt Damon, whom he has known since he was 5, and who happened also to be nearby, in Pittsburgh, making a different film, “Promised Land.”

“He came over for dinner, and he was bummed out,” Damon told me. “Casey has always been very hard on himself. But he was 35, he’s not a kid anymore and he was genuinely disturbed about what had happened. He had just done a scene with Bale, and he had been so blown away by what Christian had done — and it wasn’t anything remarkable; it was a difficult scene, but what Christian had to do was relatively simple, he wasn’t pulling any tricks. He was genuine and honest and real.” According to Damon, Casey told him that, by comparison, “I was just pulling faces.”

“Casey’s easily one of the best actors of his generation,” Scott Cooper, his director on “Out of the Furnace,” said to me. “In many cases, you hear someone say, Well, this guy’s underrated. That might be true among the general populace, but with Casey, among his peers, he is certainly not underrated.” Cooper, who directed Jeff Bridges’s Oscar-winning performance in “Crazy Heart,” had thought of Affleck specifically for the role of Rodney Baze Jr. — a P.T.S.D.-addled vet who turns to drinking and bare-knuckle boxing as a way of paying off gambling debts and grappling with his inner violence.