"Yes" person
A "yes" person is defined as a man who is not able to deny, refuse or just confront for the different reasons. Sometimes, this person is a right-hand of a powerful guy who does not want to stand confrontation. Usually, "yes" people do not make anything wrong to others, as they treat themselves bad first of all because of the lack of self-confidence. Anyway, in the team work they are able to trick everyone. For example, you are not aware that you are talking to that type of people, but you are going to ask him/her for something to do. As a result, you a get a pile of promises, that will not be completed. The reasons for promising are different. He/she is afraid to make you angry or disappointed with the negative reply. They do not like to make troubles and rock the boat. They do not like the troubles at all. It is easier to promise than to explain that the task performing is impossible. Passive behavior is their reply to your active demanding.
So, the bad news is that such people are not able to act independently, produce decisions and control themselves. As I know, it is quite hard to change such people's psychological status; they are too stuck to self-definition. It is useless to shout on them, blame or suppress. The good news is that such people are good co-workers under an energetic and critical leader. What should you do in order to cope with that? Give a usual task, but explain what to do in details, show the way, give the light in the end of their tunnel, organize him, and make him work towards fulfilling the plans. Your authority is to influence such people. They are able to do everything possible in order to get your satisfied with their work.