Demand characteristics are one of the so called experimenter effects in behavioral research. They describe the tendency of interview /usability test participants to give you (as the experimenter) what you want based on what the participants think what you might expect from them.
This doesn’t require that you tell them what you want explicitly. Participants only need to guess/hypothesize what you want from them based on some subtle cues you give which are often based on your implicit opinions. So the assumption your participants make regarding what you might want from them is sufficient enough. For example, a general assumption which occurs during usability testing sessions could be that you want them to like the product you’ll be testing.
It’s important because demand characteristics may put the complete validity of your usability test or interview at risk.
How weaken/temper the effect:
1) Make clear at the beginning and throughout your test or interview that you want to hear honest feedback. Try to take the fears of non-desireable answers away.
2) Be aware of subtle cues/non verbal language your participant sends out. Seems the answer forced? Is he/she struggling? Take notes during the test, ask afterwards in the debrief interview, eventually play the scenario through again.
3) Last but not least: Demand characteristics are one of the reasons the designer, who actually designed the system is not the right person to conduct a usability test or doing debriefing interviews, because he or she may send subtle positive cues about the system he/she designed and so participants might react more positive than they actually feel about the task or question you gave them.
So it’s better another person is conducting the test or interview, in best case people that are not directly involved in the design or development team. If this is not possible, it’s important to be aware of this bias throughout your test.