In my interview, they did a skin check to see whether I had any scars (which I do, one on each hand) but they weren't bothered with it. This was done in the personal interview! Teeth check, what would that include?
I think the procedure is different when it's an open day opposed to a recruitment campaign. If it's an open day, you have to submit your application the first day and they will screen it. If shortlisted, they invite you to come following day and that's where Stage 1 and Stage 2 come into the picture. If it's a recruitment campaign, they would have done the shortlisting back in Doha and invited you (you need to bring with you the invitation because otherwise they won't allow you in). Straight into Stage 1 and Stage 2.
And yes, it might be a good idea to make conversation with the recruiters when you are submitting your application, rather than being silent, not saying anything. Ask them how they are doing, if it's their first visit in Singapore, if they are enjoying the city etc...