During lunch, in addition to finding out more about your legal background and experience, the interviewing attorneys may also try to gauge whether or not you are someone they would like to work with at the firm on a personal level. Be prepared to discuss interests outside of practicing law. If your interviewers feel that you are not a well-rounded person, they may be more hesitant to provide a strong recommendation of your candidacy to the firm.
Before your interview, think about a couple of hobbies or interests that you would feel comfortable discussing during lunch. Sports and travel are generally safe subjects to bring up during your interview. Remember: being someone who has fascinating interests outside of the law is something that many firms find very attractive in candidates.