I imagine you're going to do some sort of thematic analysis and it would be a type semi-structured interview?
I am curious as to what the aims your thesis are, and what perspectives in particular are you looking at and why? Is this specific to Final Fantasy XIV, or are you looking at MMORPGs in general or just MMOGs?

Also if people are lying you would be generally able to tell as there has been a large amount of work done on the gaming motivations. If they are so absurdly outside the norm it will be easy to tell, or if someone has something that is generally believable but fake, then that would be a limitation of the data collection method.

Sounds like an interesting project though. I can recommend reading Nick Yee's 'Proteus Paradox' and Jamie Madigan's 'Getting Gamers' for some really digestible information. Though I am sure that you have read up on a number of things in your literature review.
Good Luck!