Shortcomings

In this scene, Keyes is talking to a man known to have seen "Mr. Dietrichson" at the trian station. The actual Mr. Dietrichson did not show up at the train station. Filling Mr. Dietrichson's shoes was Walter Neff.
Walter Neff is incredibly nervous in this scene. He is a sitting duck, waiting to see if the man from the train recognizes him or not. Walter is clearly nervous, just by looking at the stance he is taking. He is standing with a protective arm across his chest, while the other arm is to his mouth, as if he is biting his nails in anticipation. His brow is furrowed and he is staring at the man who could decide his fate. Walter is also leaning against a large cabinet structure. The structure is massive, in heighth and width. Walter is a tall man and if he were to lean against something it needs to be able to hold him up, which this successfully does.
The fan in the room is still during this scene. It can indicate that the air in the room is still as well, it doesn't give any relief to Walter, who probably is sweating through his suit. The fan not moving adds to the fact that everything is waiting in this scene. The man from the train station is waiting to get out of there to enjoy his short vacation, Keyes is waiting for the man to give any hint to whether or not he saw the real Mr. Dietrichson and Walter is waiting to see if the man recognizes him.
Keyes is more of the foreground of this shot while Walter is the background. This is often done throughout the film, to create the effect that Keyes is taller than he actually is. Keyes is supposed to be the big authority figure, when in fact he is so much smaller than Walter. Also, Keyes plays a more prominent role during this scene as investigator, while Walter can't do to much and is just watching the discussion unfold.
The man from the train station was craftily placed between the two men to show that he is the separating factor. He stands (or sits) alone as the deciding factor to Walter's fate, and to Keyes' discovery of Walter's crime.