To me part of successful candid photography is to capture people in their natural element without them being aware of it. The moment they realise they are the subject of photography people start to pose.
As you can see from this portrait of the woman with red hair I have failed to capture her without her realising it. In fact she seems to be a bit surprised by me photographing her. Her look is quizzical yet she did not turn around.
How would you react if you realised you are being photographed?