Willy's boss, Howard, is a character outside the Loman family you can write about. Howard attempts to be patient with Willy but ultimately fails. Though he is stern and direct with Willy in their interview, he does attempt to be sympathetic for a while. Willy, however, presses too far against Howard's good will and is fired.
This scene is the only one featuring Howard, but in it we see an important truth about Willy. For all his bluster earlier in the play, Willy's incapacity as a salesman becomes clear. Willy complains and makes excuses, but Howard states the case simply. Willy is no longer capable of representing the company.
The way that Willy wears down Howard's good will and even shouts at Howard demonstrates a general aspect of Willy's character. Willy is proud to a degree that is completely unjustified and this pride leads rather directly to his demise.