Entering first time with DTV via Phuket.
Can confirm as other's have mentioned that they undertake a small interview, as I did it last night.
Seems not up to their discretion and each IO has been told that it's mandatory a first entry on DTV needs an interview as the officer straight away asked me "First entry DTV?" then asked me to follow him when I confirmed.
Had to complete a Passenger statement form with arrival and departure flight details, statement of funds, address in Thailand and the reason for the visit (I.e. the reason for which the DTV was applied for).
I then had to show some of the documents that I used to apply for the DTV (Supporting letter from Dental Practice for Invisalign and copy of their Medical License) as well as my exit flight booking (I had one booked to leave in 3 weeks).
He asked me a few more details about my plans and residence, as I had the DTV approved in London but I was flying in from Hong Kong and returning back there in 3 weeks, and why I chose Thailand.
After that he seemed satisfied and walked me over to a desk for a colleague to stamp me in. He explained the reason why I'd been asked all the information was because they have had such huge numbers of people entering with DTVs and lots of them don't have any idea what a DTV is, what gym they're meant to be training at if it was for Muay Thai, and don't have any documentation for the visa. I guess there's lots of people using agency's who do everything for them and sometimes fictitiously.
The name of the Dental Practice and my flight details had been written on post it notes and stuck to my passport and the IO processing my stamp then entered these into the system. It seems as if they're unable to see on their systems the information submitted to the embassy, so they're looking for clarity on how the visa was applied for on first entry so they have it noted.
Overall, didn't take that much time and as long as you have all of your documents in order it seems you shouldn't encounter any issues. IO was nice and friendly and even jokingly asked me how the milk tea compares in Hong Kong to Thailand.