Its pretty laxed..or at least mine was. Just know what your saying and dont mumble. Have a list of questions you have so you can get answers. Just be friendly and open.
Every AAP will conduct the interview a little differently.
Be ready to answer questions about your motivation to attend, how you can enrich others there, how you deal with adversity, and what plans you have in case you don't get in.
Also remember this is an interview. Dress appropriately, thank the AAP who interviews you, follow up with either by email to thank them personally and let them know when you hear something from the Academy. Many AAP’s are volunteering time and travel to meet with you and they have your best interest at heart.