Things I noticed on my tenth viewing of Brick: (with some help from a friend watching it for the first time)
Brendan never actually tells Brain that he found Emily dead. And Brain never explicitly acknowledges the he understands this.
Stoic as he is, Brendan doesn't seem upset enough at Emily's death, but his brief breakdown much later in the story makes up for this, and we instinctively know it's his grief over Emily's death.
Is Brendan faking or exaggerating his injuries in order to mislead Laura or keep he off balance? On his way to the meeting at the flood runoff pipe, he staggers away, seemingly about to collapse. Then he suddenly straightens out and walks steadily and with purpose. I always thought we were seeing his mind's resolve overcome his body's breakdown in real time, but now I think he was faking it.
One of the most sympathetic main characters, other than Brendan, turns out to be Emily's killer. This tipped me off to what should have been obvious all along. This is a true Noir film, in that it is very anti-woman. All of these guys are, to one degree or another, sympathetic, and they all come to grief or worse because of women who are either scheming or hapless.
Still, Emily explains better than any true Noir the motivation behind at least her seemingly stupid behavior and decisions ... the cynicism of her Noir man drove her to it.