When Observing Users Is Not Enough: 10 Guidelines for Getting More Out of Users' Verbal Comments

One of the principles underlying usability testing is that observing a user perform a task provides more reliable information than simply asking the user how easy it would be to perform the task. By observing users, you can assess whether they are actually able to use a product. By asking them, you simply cannot.

Peyrichoux, Isabelle. UXmatters (2007). Articles>Usability>Testing>Methods

When Role Playing Doesn’t Work: Seven Guidelines for Grounding Usability Testing in Participants’ Real Lives

Usability testing makes use of a lot of role-playing scenarios like this one, and many findings and design recommendations result from participants’ responses to these scenarios. But an over-reliance on role playing when testing a product and making design recommendations can have major downsides and risks

Peyrichoux, Isabelle. UXmatters (2008). Articles>Usability>Testing>Personas