don't think of it as an interview. You are basically presenting documents. You had your interview with the employer.
The
TN letter should state the category of your TN, and the job duties, which should closely match your degree. It should be clear that the school is post-secondary.
Then you present your degree, and, if your degree is unrelated, proof that your course load was overwhelmingly geared towards the job duties you will have.
Your passport will prove you are Cdn.
There really should not be anything to explain. If something is missing from the letter or your docs, they will ask you -- not to explain -- but provide better docs. You're not going to the border to teach them about your job, you are there to prove by documents that you, your job and your employer are TN-eligible.