I'd just stand there and smile. And when the person is within talking range, I'd decide what to exactly, based on how hostile he/ she is. More than likely, I'd just introduce myself and say that I am an amateur photographer who very much enjoys producing artistic photos. Then I'd go on to comment why the person is interesting and eye-catching, that he/ she is a unique part of a picture I had in mind.
I'd even show the picture on the preview screen and say that I can send over a copy if I can have an email address.
And then if I were thick-skinned enough, and the person was very friendly and accepting, I'd say that my first photo was not perfect, and if they would help me with a pose for another shot, it'd be great.
On one occasion a construction worker caught me taking photo of the worker's quarters, specifically a messy scene with coats, yellow helmets on a rack and tins of paint lying around. He asked me in Chinese, "What's so interesting that you have to take photos here?" Then I told him how I liked the colours and how this arrangement mirrored the lives of workers. They are tough, live it rough and are hardworking. And I even asked for permission to take his portrait after which.
The only thing I regretted was not trying for the perfect shot of the man. If I'd just asked for permission to take his photo next to the rack, instead of standing in the doorway, I'd have had a nice photo.