If you are trying to incorporate usability testing into your development process, you might be lucky and have someone else available who can arrange and run the testing sessions. That’s what my colleague Jenny has done here at the EBI for one of the major projects going, but she can’t be everywhere.
It isn’t an ideal situation perhaps, but very often the role of usability test facilitator may fall to someone from the development team itself, for simple reasons of funding and available manpower. If this is the case, you have to learn to be detached; to encourage the test participants to be as vocal and critical as they like; you need to develop… a poker face. Stick to your script, let the user work through the tasks, and do not intervene!
Sure, if participants get totally stuck and frustrated, then you can guide them, but resist the urge to turn a usability testing session into a teaching opportunity.