Don't make this hard. Be polite, but assume the person is busy and doesn't want to wade though 500 lines of pleasantries. So be terse and get to the point quickly. I'd write email, sit on it for a day and reread. Polite? terse? Ambiguous anywhere?
Start off email formally...
Dear Dr. XYZ,
Thank you for meeting with me on (date) about (whatever). yada yada
As per our conversation I'd like to follow up with you on the following questions which you couldn't answer off the top of your head.
Of course depending on the context of the original conversion different text might be necessary here. Did the expert say he'd have to look a number up, think about it in more detail, or did the interview simply run out of time? The point is to try to politely remind the expert of the how/why of the email being agreed upon. If you asking a new question so state. Don't assume because expert agreed to look up one number that he is obliged to answer 25 new questions.
- (list questions - I'd do it with numbers so that it would make reference easy...)
Thank you very much for your time. I look forward to your response.
Regardless if they sign off the email with just their first name stay formal unless specifically asked to use first name in subsequent emails.
Point about numbering questions is to be able to say I didn't quite understand you answer to question 4. (whatever you didn't understand...)