Explain how you approach managing volunteers differently from managing employees.

Don't say that you treat them all the same. The fact that the interviewer is asking the question suggests that they expect you to be able to articulate a difference in how you manage the two groups.

"Volunteers and employees are motivated very differently. Employees represent the organization officially and legally. They are paid to do a specific set of tasks, and hopefully, those tasks are a fulfilling step on their professional journey. Volunteers can have a significant level of contact with our clients, but we have few measures to hold them accountable. I have found the best approach, when possible, is to appoint a volunteer coordinator to train and schedule volunteers and work directly with them to keep them connected to the organization's mission and interested in continuing to help."