From the point of view of some people in the client organization, the consultant is seen as a threat. In these days of outsourcing, their jobs may be threatened. Having a consultant doing some work may be an implied criticism of internal people. None of these things may be true but these perceptions often exist in the client organization. In order to be effective, these things are to be understood and handled.
One thing that often surprised me was how much power the outside consultant was perceived to have by people in the organization. This can help with the project or may hinder the effectiveness of the consultant. The key is that these dynamic are always present and the consultant must learn to deal with them is a way that increases the likelihood of a successful outcome.