Dear…
Please allow me to extend my appreciation to you for allowing me to interview you recently. It was a great opportunity for me to learn more about your company and to better understand your line of business. I actually found it to be quite educational.
In particular, I was fascinated to learn that __(briefly describe an area that you found fascinating)__. I have to say, I certainly hold a whole new respect for the complexity of the __(type of industry)__. You __(do run/are part of/etc.)__ an impressive __(factory/company/etc.)__.
Thank you again, __(Name)__, for affording me __(length of time/i.e. an hour)__ of your time. It was a most enlightening experience.
Sincerely,