Be prepared by practicing answering questions that are of course related to the job. Some of the sample questions I've encountered in the past are:
What Certifications do you have?
How do you keep your fitness knowledge up to date?
Why do you want to be a personal trainer?
What experience do you have with customer service?
What is your personal experience with fitness and working out?
Are you CPR certified? When does it expire?
Tell me about an experience you had serving an unhappy customer. What did you do?
The trend on teh questions start with BASIC Facts like your certifications, years of experience and the like. As the interview progresses, the questions become more situational so you should prepare and anticipate this type of questions.
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