Ask HN: Paired programming interview, what should I know?

I have a programming interview coming up. I will be paired with someone more senior from the organization. It's my job to identify a small feature or refactoring that I would like to do to a code sample I previously provided. The interview will take ~30 minutes and I've been told that ideally the feature should only take 10 - 15 minutes to implement.

I've never paired before, so I'm not entirely sure what to expect. Further, this interview is for placement in a group for experienced programmers--not for a job interview. I'd love points on how to prepare and things to do / not do during the interview.