The mask behind the mask: more than the inside softball of hagiography, Ricki Stern and Anne Sundberg’s “Joan Rivers: A Piece of Work” is as forthcoming as the veteran performer’s rudest, funniest jokes. With a work ethic like few others, Rivers continues to work small clubs night after night at the age of 75; one of her greatest fears is the white of blank spaces in an appointment book. Rivers’ feelings about her looks, her body, the desire to still be loved, is expressed with unceasing fury. Does she work blue? And how. But some of the most touching material involves the setbacks she’s suffered, from losing the mentorship and friendship of Johnny Carson to the suicide of her husband. Stern and Sundberg’s earlier films include the wrongful conviction doc “The Trials of Darryl Hunt” and the Darfur genocide film “The Devil Came on Horseback.” What makes the pair suited to following this comedian around for fourteen months? Observational skill, compassion, distance. They’re stellar ethnographers of this tribe of one. There are surely more layers, yet the woman who’s revealed here is a brash intelligence whose days as a red carpet-auction channel hawker are only a sidenote, not the centerpiece, of what she’s accomplished across a long lifetime. Plus: this is one blazingly funny film. Even at her age, it’s still id stuff. 84m. (Ray Pride)