Last summer, I was getting ready for the most important job interview of my career — one with Assurant’s Board of Directors for the role of CEO and president of the company. While preparing for the interview, I relied on the lessons I had learned some 30 years earlier when I was just starting out. Like most worthwhile lessons in life, they were acquired the hard way.

I was in my first year at Harvard Business School and had lined up interviews for summer internships at all of the major consulting firms. I was excited about the work I’d be able to do and what I’d be able to learn. I was confident I was qualified and figured I would easily get a job offer, even though I hadn’t even been interviewed. I had a strong academic record, aced the practice case studies and was passionate about the field. The only thing left to do was to meet with the partners at the firms for my interviews.

That summer I found myself unemployed. I had struck out with all of the firms. I had never failed at this level before and wasn’t really sure what happened. How could I have messed up so badly?

It turns out I still had a few things to learn.

During the interview process, I hadn’t distinguished myself from any of the other graduate students, and that hurt my candidacy. I was passionate about being a consultant, but the companies were largely indistinguishable to me, which was clear to those on the other side of the desk. I didn’t know about their corporate cultures. I didn’t know the differences between the major firms, what their strengths were compared to their competitors, or what they valued in employees. The interviews and resulting rejections impressed upon me the importance of being prepared and having knowledge of and passion for the company you want to work for.

First, be prepared. Many candidates rely on a superficial level of knowledge about a company when they go into a job interview. Dig deeper and be curious. Learn the company’s history, its strategy, its customers and the direction it’s headed. Spend the time necessary getting a good grasp of what makes the company unique in its space. Understand what your skills will bring to the company and whether or not you’ll be a good fit based on the company’s culture.

Second, be passionate. Credentials aren’t enough to differentiate you from your competitors. If you are passionate about the role, the work and the company, that will be visible to everyone you speak with and will give you an advantage. I once hired a college graduate who had majored in Latin because she took it upon herself to revise a Latin textbook she thought needed to be updated. Many people might have questioned the wisdom of hiring someone with a non-traditional background, but from my point of view, you can’t instill that level of passion into someone. Employees who are truly driven to make a difference are the ones who will help the company achieve its goals.

I’m often the interviewer these days and look for these qualities to separate the good candidates from the great. But last summer, I had the opportunity to put them to practice when I interviewed for the chief executive role at Assurant (AIZ). There were a lot of strong candidates for the job, so I knew I’d have to differentiate myself from the others and answer some very tough questions. I had to share the company’s vision, but more importantly, show passion for its history and employees while demonstrating a clear understanding of the culture and values. While I counted on my experience and background, I set time aside to prepare. I thought about answers to the questions I could be asked and considered what I might bring to the role that would differentiate me from the other candidates.

There are many things you can wing in this world, but a job interview shouldn’t be one of them — not if you want the job. It is a disservice to you and the person interviewing you. You lose out by not learning whether the position or the company is an actual fit for where you want to take your career. The interviewer loses out because not only have you wasted his or her time, but someone else better fit for the role may have missed a great opportunity.