Dawn Dugan specializes in writing books, articles, white papers, reports and special communications for the human resources industry. She is the co-author of Effective Executive Compensation, which was published by AMACOM in 2008. Read more...
"That's a tough one. I can't remember working with someone I've found truly difficult. Inevitably there are interpersonal challenges at work. None in particular come to mind, but as a general rule I've learned from these situations and am grateful for them."
Spend a little time thinking, acknowledge that workplace conflicts do occur, then state that no one particular co-worker really comes to mind.
The goal here is to show that you are flexible, easy to work with, and not afraid to deal with conflict.