Everyone seems to wear period clothes at the castle as the bored occupants toy with each other: the owner Curt Jurgens is married to husky Monica Vitti, whose incestuous dandy brother is also to hand- a typical role for Jean-Claude Brialy. Jean-Louis Trintignant is the student who visits and soon he and Monica are exchanging long lingering glances, to the chagrin of Brialy. There is also Suzanne Flon as Curt's sister, and Sylvie as the old grand-mother, and also - though I don't know what she is doing here - is French pop girl Francoise Hardy.
Now,we like Francoise a lot (see label) but she is hardly acting here. Monica gets to do a bit of comedy after those Antonioni roles. It is set in winter so the castle is surrounded by snow, It is obviously a play though as people sit around and talk a lot.