There is a good balance between direct and indirect character exposition in this play. Through various means, characters are developed psychologically and emotionally so that we do have access to the most salient features of each main character in these areas.
Perhaps Happy remains mainly defined indirectly, however even he is given a chance to "have his say". He describes himself early on (in his flaws, his disappointments, and in his strange pride) and reiterates these qualities again, less directly, in the requiem.
Biff and Linda all articulate their inner lives repeatedly through the story. In this way, we see Linda's constancy, her vehemence and hard choices. Regarding Biff's character, it is Biff's own commentary on himself that offers a final and full articulation of his psychological and emotional state. This dynamic also helps to show how Biff changes in the story.
Willy, a more complex character than the others, is developed in more ways than the other characters. He disagrees with himself often so that even the things he says about himself must be seen as only indirect characterization. Yet, the "reality" becomes painfully clear in his moments of delusion and in his suicide.
Though Willy is arguably incapable of truly or accurately characterizing himself, other characters comment truly and accurately on his character at many points in the play. His persona and his inner life are each rather fully available to the reader.