Usability testing and research is a core skill of user experience design. The Nielsen/Norman Group are the gurus of user experience research. Their website has many helpful articles providing advice and research on a wide range UX topics.
Here’s an article I really like, First Rule of Usability? Don’t Listen to Users. This may at first sound rude, but the message is to observe what users actually do, and not what they say they do with a tool or user interface. Nielsen writes another article Field Studies Done Right: Fast and Observational that backs up this practice with additional pointers on how to communicate with the users who you are observing in your research. Following up on how to conduct good usability tests, User Testing is Not Entertainment, provides do’s and don’ts of conducting a study.
This is not to say there is no room for talking with user study participants. Nielsen advises how to interview users to gather valuable information in Interviewing Users. Another article by NN/g member Kara Pernice suggests three good techniques, Echo, Boomerang and Columbo for Talking with Participants During a Usability Test. Nielsen recommends having users think out loud while using software, and provides a video example in Demonstrate Thinking Aloud by Showing Users a Video.
UX researchers should always behave ethically and respect the participants in usability tests. Team Members Behaving Badly During Usability Tests defines a number of guidelines to keep in mind during usability testing. A good summary on the topic of user testing is found in UX Without User Research is Not UX.