This is your opportunity to (1) show you know a little about the bank and (2) kiss the ass a bit of the person with whom you are interviewing. Just don’t go overboard with #2.
If you have friends that work for this bank, say so, and mention that they are really enjoying their experiences. If you are interviewing with a bulge bracket bank, mention how you are excited about the prospect of getting a broad experience and learning about different products or industries. If you are interviewing with a boutique, talk about how you like the idea of a smaller firm, where you might have more responsibility and more interaction with clients and senior bankers. Without a doubt (unless this is the first person with whom you’ve ever met), state how you’ve really liked all of the people from this bank that you’ve met before.
If you have previously had the opportunity (for example, in prior interviews or at recruiting receptions) to ask other bankers from this firm (or better yet, this particular interviewer) why they like working at this bank, then by all means recycle these answers! If they say the culture is great, you say you want to work here because the culture is great. If they say dealflow is strong, you say you want to work here because the dealflow is strong. You get the idea…