Jerry Weinberg wrote a number of books to this point, starting with 1971's 'The Psychology of Computer Programming.' Here's what he had to say a decade or so later...

"The First Law of Consulting: In spite of what your client may tell you, there’s always a problem.

The Second Law of Consulting: No matter how it looks at first, it’s always a people problem." [0]

Everything he wrote is worth the time to read.

[0] Weinberg, Gerald. "The Secrets of Consulting: A Guide to Giving and Getting Advice Successfully", 1986

Saw this post title and immediately thought of Jerry.