At first, during the 1st and 2nd act, the intentions of Marthaler was not apparent to this observer. The singers kept a very detached stature overall. At the end of the 1st act, for example, apprehending the approach of King Marke the four (Tristan, Isolde, Brangane, and Kurwenal) try to compose themselves by sitting stiff in geometrically arranged chairs.
The contrived and restricted manner in which everyone acted transfigured into significant and moving meaning, as, at the end of the 3rd act, after singing the final words assigned to each, first Kurwenal, then Marke, and finally Brangane turned away from the world, standing in an upright position, face to the wall.
Isolde herself, after singing the beautiful Liebestod alone in the bed that Tristan has been lying, finally lies on the bed, covering her body and face with the white sheet.
So the opera ends with people encapsulated in the isolation of each, distanced from other people, no matter dead or alive. And the love itself, glued by the poignant word "und", is emancipated and lost for ever in the ocean of eternal solitude.
Image from the final act of Marthaler's Tristan und Isolde, from
Wagneropra.net