Be prepared to explain how you could tailor different exercises to different people. Sure, you can guide someone learning how to bench. But what if that person can't even lift the bar? Can't get back up once they lie on the bench? Have a hard time walking?
Interviewers don't care about what sports you played. They are looking for people who put the 'personal' in personal trainer. Think about how intimidating it can be for someone who finally worked up the courage to start exercising. It's your job to make them comfortable with this new environment.
Prepare by putting yourself in a clients shoes and thinking about what you would want in a trainer.