To promote its new fall drama "Person of Interest," CBS Corp. is turning its eye on viewers.

To highlight the show's theme of citizen-surveillance, CBS is tapping into the trend of interactive billboards, installing one each in New York City and Los Angeles. The window display looks like a mirror, and when passersby stop and turn toward it, sensors zero in on their faces with the notification, "Person of Interest Identified," followed by "Taking Photo."

After...