Change is coming. Leadership at the top is being redefined as boards take a more active role in decisions that once belonged solely to the CEO. But for all the advantages of increased board engagement, it can create debilitating questions of authority and dangerous meddling in day-to-day operations. Directors need a new road map--for when to lead, when to partner, and when to stay out of the way.

Boardroom veterans Ram Charan, Dennis Carey, and Michael Useem advocate this new governance model--a sharp departure from what...