When I was a lot younger, I worked as a photographer for a University newspaper that published three times a week. I shot a lot of photographs of people without first asking permission.
My favourite response? -
It wasn't strictly a "Street" circumstance, but it was certainly a photograph taken without warning.
It was taken at a well advertised and very well attended talk in one of the largest auditoriums on campus.
The speaker responded to my impromptu photographing of him with a big wink.
It was Yousuf Karsh