There are two possibilities for a Thank you Letter after Interview! The first would be from the Company Official that conducted the interview. The second would be from the person that was being interviewed. In both cases it is very important to write a Thank You Letter!
Let's first take a look at it from the perspective of the Company Official. Of course we would not consider writing the letter until we know that the potential candidate was not being selected. However, in some cases it may be the right thing to do even if you're unsure as to whom you will select. As an example if your selection was going to take an unusually long period of time to make. Then it may be appropriate to send out a Thank you Letter after Interview!
I wanted to thank you again for taking the time to meeting with me to discus~ your professional background and experiences. I truly enjoyed speaking with you and getting to know you better.
If there were any other questions you had or information you wished to share, please don't hesitate to contact me!
Again, John, thank you for your time and I will contact you as soon as we have made a decision.
If you have already made your decision and the candidate was not selected you could possibly send your Thank you Letter after the Interview like this;
I wanted to thank you again for taking the time to meeting with me to discuss your professional background and experiences. I truly enjoyed speaking with you and getting to know you better.
We unfortunately did select someone else that we felt was a better match for our organization; however, I would encourage you to continue your search.
Thank you again, John, and I wish you nothing but success in all your future endeavors.
Now let's take a look at what a candidate might want to send the Company Official that just interviewed them. I would encourage and recommend that you send this almost immediately after your interview. I would also suggest that it be hand written on a greeting card, i.e., a Thank You Card!
Just a quick note to thank you for taking the time today to meet with me! I was extremely impressed with your organization and I look very forward to hearing from you soon.
Again, this type of thank you letter should be hand written and on a Thank You Greeting Card. The greeting card should be somewhat generic in nature! Do not misspell any words and try your best to make your hand writing legible.
Good Luck and May God Bless You!