You’re onstage, in a play, in front of an audience … and you don’t know your lines. Your fellow performers stare at you expectantly, waiting for your half of the conversation. For most actors (and Christopher Durang), this is the ultimate nightmare. For an improvisational comedian, it’s an opportunity. That’s the high-wire act of Gravid Water, the much-beloved Upright Citizens Brigade show that puts professional theater actors (who know their lines) in scenes with improv artists (who don’t). The funnyfolk don’t even know what play they’re in — or, for that matter, whether it’s (gulp) a musical. The format, created by Stephen Ruddy, is more or less guaranteed to yield laughs, and often a fresh dramaturgical unpacking of a chestnut, and it regularly attracts top-flight talent on both sides of the theater/improv divide. (Frequent participants: 30 Rock–ers Scott Adsit and John Lutz.) Gravid Water is performed the final Monday of every month, and there’s often a surprise guest. (A certain Hall Pass–holder and SNL varsity senior is rumored to be dropping by tonight.) Adsit and Tony nominee Maria Dizzia are already booked, along with Becky Drysdale, David Kenner, Tara Copeland, and many more. The show tends to attract the most thrilling and inventive risk-takers of both camps. If last night’s Oscars felt a little too staged and staid for your tastes, I highly recommend Gravid Water as a corrective tonic.