You need to maintain the confidence of your previous employer/client, while giving the interviewer enough information about your skills and experience that they can make a hiring decision.
One good way to do this is to blur out the details of the specific project (to whatever degree you feel comfortable), while providing specific details of the value that you brought to the project.
An example could be something like this:
"I advised a major consumer electronics on a significant new product launch. Worked closely with the VP of marketing to segment the target customer base and develop an overall marketing plan to maximize long-term ROI. In order to do this, I gathered information from multiple sources (list sources), developed an initial hypothesis, conducted focus groups to refine the hypothesis, and modeled scenarios across 4 different variables"
This would probably lead to follow-up questions about my specific responsibilities and activities, but not about the project itself.