Giggling can be a response to stress, and Matt Smith is 22 years old. IMO, being so young can certainly be a reason for not being able to keep one's emotions in check.
I thought that he seemed to be completely sincere. The occasional giggle did not bother me at all, it was not a reaction to something humorous or secretive, playing a prank, what have you.
I would not dismiss what he has to say, I assume that he has filmed many people and I look forward to seeing what he has recorded.
There is not a whole lot of difference between laughter and tears in some cases.