Josh, protagonist of Jake Brasch’s The Reservoir, is a New York University drama student and veteran blackout drunk. Careless and self-centered, Josh spreads pandemonium wherever he goes. Irksome as this conduct may be for those around him—especially his long-suffering mother (Heidi Armbruster), Josh is an audience charmer. Credit for that goes to Brasch’s wit and an adroit performance by leading-actor Noah Galvin. Yet the achievement of this production owes less to the comic capital of the central character than to the heartfelt depiction of Josh’s grandparents, embodied by four notable veterans of the New York stage: Caroline Aaron, Peter Maloney, Mary Beth Peil, and Chip Zien.