I don't think it can be emphasised enough just how relaxed all of the troops where. Although I don't doubt they were ready for combat they clearly weren't expecting any trouble.
This man was one of the soldiers at The Ananda Samakhom Throne Room and was continually smiling.
I can imagine that they would all have been pretty worried about the possibility of having to fight within the crowded confines of their capital city and much of the good nature is probably relief at the bloodlessness of the outcome.