Americans know Vanessa Paradis, 39, as Johnny Depp’s semi-spouse, from 1998 to this June, and the mother of his two children. In her native France, though, she has been a pop diva from the age of 14 (with her 1987 hit “Joe the Taxi Driver”) and a movie star when she feels like it, in such pre-Depp films as the sexy Élisa and Patrice Leconte’s The Girl on the Bridge. She does her most demanding and mature work in Jean-Marc Vallée’s French-language drama about two seemingly unrelated couples: a mother and child in the Paris of the late ’60s, a divorced couple in today’s Montreal. As Jacqueline, caring for her Laurent (Marin Gerrier), who has Down syndrome, Paradis acutely demonstrates how love can accelerate into an obsession that the beloved is unable to fully, eternally reciprocate. In this mystical treatise on such obsessiveness, Paradis is the goddess of a devotion both exalted and terrifying.