Methods for Successful 'Thinking Out Loud' Procedure
http://www.stcsig.org/usability/topics/articles/tt-think_outloud_proc.html

Ramey, Judith A.
STC Usability SIG
1998

Abstract:

When you are screening the participants for a study, notice how they respond to your questions. Decide on a strategy for engaging the participant before they arrive for the usability study. Be careful of the social dynamics you set up with the participant.