How do you interpret this?

[Jennszoo]

Applicants should also be prepared to participate in an on-site interview with department faculty. A writing sample will be requested of each applicant during the interview process.

Would that mean to bring a writing sample to the interview or would they do some sort of GRE-like thing where you produce a sample on demand?

[ColorlessGreen]

I imagine they mean they will request a writing sample before the interview, so as to ask more involved questions about your research, interests, competence, etc. However, I'm not that acquainted with the process for your field, so I couldn't say for sure. I do doubt, however, that any committee would make you write something on demand in the middle of an interview. I hope they wouldn't be that insane.

[Lauren the Librarian]

Would that mean to bring a writing sample to the interview or would they do some sort of GRE-like thing where you produce a sample on demand?

I have no idea about grad school but when I got called in for an interview at a small, liberal arts university for undergrad, I was put in to a room and told to write a paper in 60 minutes with no warning of the topic before hand. The invitational letter has a similar statement of "writing sample will be requested during the interview".