Take this for what it's worth as I haven't done this before. You aren't exactly working for free, your working to gain experience and knowledge. That being said, I would interview them as much as they are interviewing you. I would start with researching the local sampling of photogs. See their work and get to know their reputation. I would personally want to work with someone who is established, been in business for a number of years and has a good following to gain some business experience. Also, I would want to work with someone who's work is exceptional instead of just average/mediocre to increase my skills. Hopefully you can find someone w/ both those qualities, but not always. There is one photographer in my town that meets those criteria, and the others have one or neither of them. After narrowing down the list, I would put together a portfolio of some nice prints and see if any of the top three are interested in meeting with you. From there, I would think it would be based on personalities. Do you like them, and vice versa. Can you learn something from them? You need to come to the meeting with a list of reasons why they need a 'tag-a-long' and what you can do for them. Even if your free labor, there are a lot of people out there who are just too much trouble (too exhausting, emotionally unstable, undependable, narcissistic, neurotic, etc) that it's not worth the hassle. So you need to present yourself as if you were interviewing as a perspective paid employee. I think it's a lot of common sense.