Finally, make it clear to them that if they ask you a question you might not answer it as you
want to see how they progress without external help. For me this is always the hardest part
of a usability session, when do you help the user and when do you let them struggle. My
advice here is when they hit a problem let them struggle for a minute or two, if they don't
solve the problem themselves then make a note and tell them how to achieve what they are
trying to do and be quiet again. If you are running a series of user sessions and people keep
hitting the same problem then we often prompt them after 20-30 seconds explaining that
we know this area is a problem already.