Casey Affleck’s big break has been years in the making, but he still seems overwhelmed by the way his career has accelerated in just the last three weeks. “You get ready for it, but it’s sort of surreal — that to everyone else, all these movies hit screens at the same time, and it looks like I haven’t been in anything and then suddenly you’re everywhere,” Mr. Affleck said. “And then you start hearing these questions about being a ‘leading man,’ and you’re not quite sure what to say. Because for me, it’s been a steady buildup, but overnight everyone now starts paying attention.”

Not that his smaller, earlier roles haven’t garnered attention. Known previously to mainstream audiences for his sly performances in the “Ocean’s Eleven” franchise — as one of those two rude brothers who round out the crew (numbers 10 and 11) — and to art-house audiences for his brave turn in Gus Van Sant’s cerebral, anti-narrative epic “Gerry,” Mr. Affleck has gradually built a noteworthy catalogue of atypical movie characters. He even stood out in the calculated “Ocean’s Thirteen” as one of its more realistic and less sanitized pretty faces.

But in the days since the Toronto Film Festival, Mr. Affleck’s world has changed profoundly. Following his starring role as the gunslinger Robert Ford — named alongside the most famous outlaw of them all in “The Assassination of Jesse James by the Coward Robert Ford” and paired with one of the world’s best-known actors in Brad Pitt — Mr. Affleck takes a bold step forward in “Gone Baby Gone,” helmed by his older brother Ben in his directorial debut and opening in movie theaters next week. A back-alley missing-child procedural that quickly explodes into a larger character study of a private investigator who refuses to turn his back on a case the cops call closed, Mr. Affleck holds his own against the likes of such veteran character actors as Morgan Freeman and Ed Harris.

Yet he’s the first to dismiss the notion that there’s a preconceived method to his choice of roles, a systematic progression away from the vulnerable young men seeking approval in “Ocean’s Eleven” and “Gerry,” as well as the inexperienced adventurer in “Jesse James,” to the amateur detective who matures and hardens quickly in “Gone Baby Gone.”

“There wasn’t anything intentional going on,” he said. “But if you take them separately, I can see the comparison. Patrick, in ‘Gone Baby Gone,’ definitely cares a lot less of what people think of him. He has a much more cynical and detached view, while Robert Ford was almost dependent on what people thought of him, which is why he wanted to be like Jesse James in the first place, to get acknowledgment and respect. But I never choose two roles because they both possess the same set of qualities. I can’t see any A-to-B progression, and I think I’d be thinking about the wrong things if I read a script and was only focused on things like that.”

In fact, his move to center stage in “Gone Baby Gone” was not something he sought out, but a leap he almost refused. “I was shooting ‘Jesse James,’ and Ben came up to visit me on the set in Canada and gave me the script, and I was really careful about that decision because I wanted to make sure it was actually something I felt like I could do,” he said. “You don’t get many chances if the first movie’s not that good, so I knew how important this was for Ben and I didn’t want to take the part and then not know what I was doing, or not bring to the movie what’s needed.”

But as the film’s production moved into increasingly dark territory — the similarities between this fictional story of an abducted 4-year-old named Amanda and the true-life drama captivating Britain surrounding the disappearance of young Madeleine McCann in Portugal, have forced Miramax Studios to delay the film’s British release — Mr. Affleck said his relationship with the director became essential, and helped the bleak and brooding story strike the perfect pitch. For starters, both Afflecks are intimately familiar with Boston, where Dennis Lehane set the novel on which the film is based. Mr. Affleck said he could connect instantly with the way his older brother wanted to represent the city, familiar as he was with the same experiences that informed Ben’s vision and sharing the same sensibilities about Boston’s seedy underbelly and back-alley characters.

“Having those same experiences and 32 years of history together, not to mention all those years of watching and talking about movies together, it put us on the same page right away,” Mr. Affleck said. “So many times, I knew what he was getting at before he opened his mouth, which isn’t to say we didn’t disagree, but we could disagree and come to a decision faster. It made everything so much more efficient.”

Mr. Affleck said he views his newfound fame — and his “leading man” name recognition — less as a triumph than as another challenge standing in the way of crafting convincing, cohesive performances. “People say ‘leading man,’ and I don’t even know what that means. It seems to be a burden, something of an albatross that you have to shake off because you want to be able to do different things, not spend all your time emulating someone else’s idea of what a ‘leading man’ is,” he said. “The only real tangible difference between a leading and supporting part is that one is in more scenes, but it doesn’t change the way I approach a character or think about this person. And if you think about it, every movie is little more than two hours in a character’s life, and if you’re more focused on being a ‘lead’ than in bringing that character to life, it doesn’t serve the character or the performance — or the audience.”

ssnyder@nysun.com