Keep Data Pure During On-site User Observation

I am going to be running small on-site user observations for UX research for a mobile application that uses voice.

I have never conducted a user observation session, I've read a lot of articles on NNGroup and other UX and usability sites. I've read how to run a User Observation session like this article and a course on UX research and data on Lynda.com but nothing has given me a conclusive or comfortable answer to: How to / should I prompt and guide the user to use the product during a on-site visit?

If I want to see how they use the product after a sales call, should I prompt the user to use specific features of the app or should I just watch what they do with the app without saying a word?

If they ask me how to use a feature should I tell them? Of course this is based on what the observation is trying to achieve. I want to see how a sales person would use the app without prompting them. But let's say they've never used the app and have no context - do I provide this?

I can imagine a user would get frustrated if they had no idea what I expect of them. If I tell them "I want you to add details on your client to the app" is that interfering with the study? I want to see if they add details to their client and what troubles they get when they try to do so.

I know some of these questions are a bit subjective, but keeping data honest is important and I am looking for the best way to do so.