In an interview with Steve Krug, he gives a great definition of what the work of an interaction designer is:
"I sometimes think the best analog for my job is a "show doctor"--the person who comes in while a Broadway show is still in out-of-town tryouts, watches the whole thing, and says, "I think it would work much better if you moved the cowgirl dance number to the start of the second act, and killed the love ballad altogether." I'm a fresh pair of eyes (which is vital, because if you've worked on a web project for any length of time, you know it too well to see it clearly) and an outsider who can advocate for the interests of the end user because I'm not involved in the internal political struggles."
Steve Krug is the writer of the book "Don't make me think!"