Source: This article was originally published on CNBC.

Written by: Courtney Connley, CNBC Make It

Nailing an interview is hard, especially for your dream job. That’s why regardless of how qualified you may be, you never want to make the interview mistake of acting like you’ve already sealed the deal.

In fact, it was this common interview mistake that cost me a job in 1981, when I was a recent Harvard University graduate.

I wanted to be a journalist, so I sent my resume to about 50 newspapers from Maine to California, and pretty soon afterward, the Kansas City Times called and asked me to come out for two days.

On the first day, my reporting try-out went well and I was told I did great. That was the beginning of trouble. I started acting, you know, a little too relaxed. Too casual. Too familiar.

I recall riding to lunch with some editors from the paper and asking, “So, what kinda food we getting?” as if they were old friends. I even met with the editor-in-chief later that day and asked him where he lived as if I was trying to spot potential neighborhoods. The next morning, I got a call letting me know that I didn’t get the job because I didn’t “fit in.”

The editor didn’t have to explain himself. I was dumb, but not so dumb that I didn’t know I had overstepped.

From that experience, I learned that you should always remember there are power dynamics at play in an interview. Even if you think you’re perfect for the job. Even if they seem anxious to get you. Even then—they are the buyer, you are the seller. Do not let down your guard. After you are hired, the dynamics may change a bit. But until then, no open job is yours for the taking; it’s yours for the winning, with skill and humility.

That’s how I approached my next interview at the Miami Herald. And, as it turns out, I got the job.

Suzy Welch is the co-founder of the Jack Welch Management Institute and a noted business journalist, TV commentator, and public speaker. Through its online MBA program, the Jack Welch Management Institute transforms the lives of its students by providing them with the tools to become better leaders, build great teams, and help their organizations win.