Fincher's attention to detail in on another level.
In the dinner scene, when the subway is passing through and shaking the flat, we can hear what might be a ball falling to the ground and maybe bouncing a bit. Later that night, when Mills is discussing the case with Somerset, he casually picks up a basketball to put it back on some furniture.
In the same scene, Mills offers a beer to Somerset, which states his preference for wine. While they're chatting, Mills fills up a regular glass with wine (instead of using an actual glass for wine). When served, Somerset is too focused on his documents to notice the glass. But when the subway passes through again, Somerset goes for a sip and gives a weird look at the non-wine glass containing wine.
I love it.