At its most profound, “Benjamin Button” isn’t about anything more important than Pitt’s very handsomeness, which, for a surprising stretch of time, is a wonderful subject for study. There is a sad scene that requires him to leave a room, and the sheer fact of how young he seems really is breathtaking. I almost gasped at one point. His acting here is entirely static and observational. It’s exactly what Pitt did as Death in “Meet Joe Black”: He just sits there. And yet this time there’s something alluring about that: He’s sitting to have a filmmaker take an elaborate photo. That’s a painstaking process, but it’s the one portrait Fincher gets completely right.