This young woman looks sad on the picture, but actually when I took the picture, she was having fun singing under the eye of my camera. She was going back and forth on the platform, and at some point she had noticed my presence. She had probably felt I was looking for something special from her. Typically a situation where my presence as a photographer had triggered something from my subject.
Anyway I am happy of the result, and we can still say it was a “decisive moment”, resulting from a particular relationship between two anonymous, one being the photographer, and the other the photographee.