Zosia Mamet and Matthew Shear in "The Boy Downstairs" (Credit: Tribeca Film Festival) “The Boy Downstairs” is a film that cannot move forward without moving backward. Diana (Zosia Mamet, most recently of HBO’s “Girls“) is betwixt and between. She is an aspiring fiction writer, who after a two-year stint in London, has just moved back to New York and is working at a high-end bridal shop.