I guess this is for a position at a gym. Here are few to think about:
-Ask about the background of the person who interviews you. This is a question that goes both ways.
-Try to read as many reviews and complaints that members or clients from that location have posted online (Yelp, BBB, Glassdoor are a few sites to visit and check for these types of reviews). If there are negative reviews out there you might want to ask why is that and what do they have in mind for fixing the issues. This is a question that you can ask if you feel comfortable about it and it’s only a suggestion.
-What would your responsibilities be
-How many other trainers they have on staff and why are they looking to hire for the position you applying for (is it because of high turnover? did someone quit and they are looking for a replacement?)
-Here is video for you to watch
I hope this helps. Just stay calm and being yourself, control your emotions and look professional. Listen to the questions the interviewer is asking you and answer them the best way you can and be honest. Show your interest for the position you are interviewing for.