If you’re a US citizen, you should bring either a copy of your passport or your birth certificate and driver’s license to the meeting. That way, if they hire you on the spot, you will be able to fill out employee paperwork (assuming they are hiring you as an employee) and you’ll have the documents they’re required to look at as employers.
Do you have a resume? If you do, bring that.
The only interviews I’ve had for trainer positions have been discussions with the gym managers / owners. I haven’t had to pretend to train someone. One person did ask me about my training philosophy and how i approached assessment, but I didn’t have to do an assessment.
When I audition for group-ex positions, I come dressed prepared to teach, in my nicest fitness attire with a professional sweat suit on top.
Bring a few intelligent questions that you have for them. You are interviewing them as much as they are interviewing you, even if you desperately need a job. For example:
1) Will I be asked to / allowed to shadow other trainers to learn from their expertise?
2) What is your sales process? Some gyms expect trainers to give a “first workout” and then sell training to the client. Some gyms do the sales separately and assign you the client.
3) Is your gym known for any specialty style of training / clientele? This is where you will learn whether they have a style, and whether that style fits you.