Mine (and others from what I've heard) was very casual! They asked why Penn, why social work, what my social work interest are/what I would want to do for my field placement, and for three personal strengths and one weakness the school could help me strengthen. The best advice I could give is to be confident in your abilities and demonstrate your passion for Penn and social work. It felt less like a formal interview and more of an opportunity to talk about yourself and interests, so talk about why you're a good fit for Penn and why Penn is a good fit for you. I hope this is helpful; let me know if you have any other questions! Best of luck!