Interim reports are necessary. Any project lasting more than eight weeks probably requires a brief, written progress report by the midway point. This report can serve as the basis for a more detailed discussion with the consultant during which you make certain that everybody involved is holding up his or her end of the job. It's an informal document, serving as an indicator of the project's general health, making explicit much of what you've probably already seen or intuited, and ensuring that the overall focus remains steady. The report will not cap or complete discussion on the project, but rather open it up.
Nothing turns out as planned.
The consulting process usually produces both delightful new information and unwelcome revelations. As a result, some changes in strategy may be necessary. But resist the temptation to respond to each layer of discovery with a new policy or revised work plan. Hold steady until you can grasp the whole picture. The challenges of working with an outside expert are many, but so are the rewards.