1st interview - get the lay of the land and see if these are people you can see yourself working with. Do you like the location? Are they well staffed? (Over/understaffed?) Are they happy? Are they too busy?
What kinds of patients do they see? Will you have to see things you don't want to treat? Can you see yourself working there with them and being happy?
Why do they want/need another person? What's the benefit to them?
On the 1st interview, do not bring up economics. Don't ask how much they make, or what you can make. Do not ask about %, partnership, etc. If they bring it up, fine. Don't you break that ice. That is all for follow-up interviews if all parties are interested in pursuing employment with you. If you bring it up, it makes you look greedy.