I would need to hear about the determining goal of the campaign (brand awareness, leads, social media followers, etc) and then planning the campaign around the desired result. Candidate will need to talk about TRACKING and using analytics (Google or otherwise) to get as much data as possible. Tell me how they're going to track it. Adding URL parameters and the like. The key to digital marketing is having as much data as possible in order to be able to leverage that data to make actionable insights. The more they can tell me about how they're going to gather the data and the ways they can look at the data to see what worked, what didn't, and where things could be improved the more likely it'd be they would get the job.