On a recent chilly night the East Hampton Presbyterian Church loomed dark and uninviting. At 6:45 a man emerged from a car on Main Street, walked around to the side of the building, and unlocked a door. Within seconds the soaring space was awash with light, and Walter Klauss, the guest conductor of the Choral Society of the Hamptons’ upcoming spring concert, doffed his coat and settled into a front pew for a few moments of conversation before rehearsal.