For others viewing this later, make a list of things you want to know about the position before you interview. Check them off as you speak with the interview when the information they give you answers a question. Ask any questions left at the end.
You should find out:
Hours you are expected to be available to work?
Can you have a set core schedule/schedule?
What other duties are you expected to perform other than personal training?
What is the dress code?
Are you an employee or contract worker?
There are a lot of other potential questions. If you can, try to ask some of the current trainers about the process. Be professional. Speak clearly and make eye contact but not creepy staring. And relax, breath, and be yourself.