Nicole Kidman is just astonishing in Rabbit Hole — subtle, fierce, brutally funny, tender when you least expect it, and battered by the feelings that hit her when she forgets to duck. Kidman plays Becca, a mother coping with the impossible fact that six months ago a car struck and killed her four-year-old son as he ran into the street. You can’t deal with that. But Becca must. So must her husband, Howie (a superb Aaron Eckhart). They try the usual routes, from God to grief counseling, but they need to carve their own path without destroying their marriage. Out of unspeakable sadness, David Lindsay-Abaire, brilliantly adapting his play for the screen, creates a movie that uses humor as a kind of healing. It’s a rough road, and director John Cameron Mitchell (Hedwig and the Angry Inch, Shortbus) doesn’t smooth the ride with glib good intentions. This haunting and hypnotic movie blinks back tears in favor of taking on the world, not running from it. Becca hilariously skewers the hypocrisy she finds in grief therapy and harangues her mother (Dianne Wiest), who lost a grown son, for comparing her sorrow to Becca’s. Mitchell directs with remarkable skill and compassion without ever going soft on the characters. Kidman, doing her best work in years, just comes at you. Her final scene with the splendid Wiest, who builds her character with uncommon feeling, is devastating. So is the movie. It takes a piece out of you.