How to observe users without seeing confidential data

My team's application lets users share files in the cloud. It's a bit like Dropbox, but our product encrypts everything to ensure security and privacy. It's intended for business users who need to let colleagues and clients see sensitive docs (accountants, financial advisors, lawyers, and government).

I don't know enough about how users actually use our product, and I want to see people in action, if possible.

I have been given the names of some friendly customers who would be happy to have me visit their office, or have a phone conversation and maybe a TeamViewer session to see their desktop. I would like to sit and watch them work for an hour or so, to see how Lockbox fits in to their work.

My problem is that these people will be working with confidential material. I am OK with signing an NDA, but I'm not sure how they will all feel about that.

Is there any practical way that I can see how users work "in the wild" without seeing their data?