So I know dress, looking her in the eye, and knowing the answers to the classic interview questions (why Wellesley, favorite book, hobbies, etc.) are important. What else should I be prepared for?
Be prepared to have some questions for your interviewer about Wellesley - not stuff you can find out on the website, though.
Ask her questions about herself, too. How did she get where she was? What did she study? How does she think We has changed in the years since she graduated? (Unless it was last year.) Ask the hard questions, too, such as whether there is a big disparity in the way wealthy and less wealthy students are regarded on campus, stuff like that.
I’m an alum who used to be very active in our club’s admissions activities. The written report alum interviewers submit is now very open-ended. Years ago we had more specific questions to answer about the applicants’ academic, EC, and family/personal interests, and also whether applicants demonstrated a strong preference for Wellesley if accepted. Concentrate on those areas, and give the interviewer great examples/supporting material to include in her report. I was grateful to applicants who left with me a resume (but no transcripts!) Also explain what your contribution to the Wellesley community will be.
Prepare answers to the “Why Wellesley?” question, and any questions regarding why you’re a good fit for the campus and what you can bring to the community.
Also, prepare some questions to ask about the school. A good question to ask your interviewer is “Why did you choose Wellesley?” Other than that, don’t stress out, and treat your interview as an informal conversation. The interview isn’t weighted heavily in the decision process, and is more of a way for you to know about the school than for the school to know about you (they already have all your information on your application). Good luck! 
Thanks for the advice everyone! I think my interview went well. We had similar interests and talked for over an hour. Although I felt it was more 60/40 of her talking about Wellesley to me answering her questions.