Written By: Big C Sometimes the most powerful performances arrive unannounced. During their To Be Free: San Francisco  residency, Sam Smith  surprised the crowd at the historic Castro Theatre  by welcoming rising vocalist Sienna Spiro  to the stage. Together, the pair delivered a soaring duet of Spiro’s breakout single Die On This Hill , transforming the intimate venue into a moment of pure vocal intensity. Smith’s rich tone intertwined with Spiro’s striking range, allowing t