The response of an overdamped bistable system to a periodic force is investigated using linear-response theory and by analog electronic experiment. It is shown, both theoretically and experimentally that a phase lag phi exists between the force and the response, and that |phi| passes through a maximum when the noise intensity is tuned through the range where stochastic resonance occurs. These results clarify the interrelation between stochastic resonance a vonventional resonance phenomena, and confirm that stochastic resonance can usefully be treated by linear-response theory.