A physician assistant (PA) provides health care and medical services for patients under the supervision of a licensed doctor. As a per diem physician assistant, you get a set payment for each day that you work. PAs examine and diagnose patients, provide treatments, order tests, and perform other duties approved by a physician. Your responsibilities are the same as those of a full-time PA, but you receive assignments in different locations, work to a fill-in PA for full-time staff members, or work temporarily when medical facilities expect an unusually high number of patients. You may switch frequently or stay at a facility for weeks at a time.