I don't think "expressionistic modes and techniques" is the correct phrase to use in relationship to how Arthur Miller's Death of a Salesman is presented. Certainly there are special theatrical techniques used in the play. These techniques are utilized because the play sets out to express the state of the main character's psyche, his inner world.
The play takes place during the last day of Willy Loman's life, and, in that brief time, much of his adult life is bared to the audience. To show what the playwright needs to show about Willy, his past and present co-mingle and intermesh on a single stage, and the viewer needs to know what is now and what was then. In a real sense, Willy Loman is the sum of his misfortunes, his soaring hopes and his dashed dreams. That's a lot to show in a few hours, so special techniques were employed in order to expose Willy's inner life to the audience. Here is the technique used, in Miller's own words:
The entire setting is wholly or, in some places, partially transparent. The roof-line of the house is one-dimensional; under and over it we see the apartment buildings. Before the house lies an apron, curving beyond the forestage into the orchestra. This forward area serves as the back yard as well as the locale of all Willy’s imaginings and of his city scenes. Whenever the action is in the present the actors observe the imaginary wall-lines, entering the house only through its door at the left. But in the scenes of the past these boundaries are broken, and characters enter or leave a room by stepping »through« a wall onto the forestage.
In addition, transitions between past and present are accomplished using lighting and music. After a brief time experiencing the play, it becomes clear when Willy is in the present and when his mind wanders into his past.