The opening sequence!
It starts on the windows and we peer out onto the courtyard. We look around, a voyeur's eye view into the world of the film. We see the fire escape sleepers and other denizens of the world. Next we see the temperature, first physically by sweat, then by the thermometer. Then comes the best part: LB sleeps by the window in his chair, we look down and see the cast. How'd he break his leg? Intrigue immediately! Pan to a smashed camera. Okay, that's a clue. Plus, we know he's a shutterbug. Moving up now we see the photo that got him that leg. He keeps a negative of a cover girl in a frame on a table.
We now know everything we need to know about our hero, the environment, the circumstances, and the theme of the film in three minutes flat. This is one of the best openings of any film ever. It tells us a little story, keeps us intrigued, gives a tiny, mini-mystery, and makes us want to know where we're going from here.