Peter Block in Flawless Consulting identifies three roles that consultants choose (or that clients choose for them):

The Expert -- the client has neither time nor expertise. You, the consultant, find out what's wrong, then fix it.

The Pair of Hands -- the client believes he understands the problem and its causes, knows what he wants done. You, the consultant, do what the client tells you to.

The Collaborative Role -- the consultant applies his skill to helping the manager solve probelms (rather than solving them for the manager.

This post was a good nudge for me to revisit Block, who makes a lot of sense and understands a lot of nuance.