When can an employee also be a ‘volunteer’?

Q. "We have an employee who works for our city theater on a part-time basis. The employee is also a volunteer for a non-profit organization that sometimes uses the theater. There are times when the nonprofit group must use their own volunteers to run the theater equipment. If our employee, when working as a volunteer for the nonprofit, runs the theater equipment, does the city have to pay the employee?" -- M.B., Nevada