A look of defiance and unrest. You have been caught. That person has noticed that you are trying to snap them and they just don’t like it. They may even speak to you about it.
It can feel uncomfortable for both parties.
But, at other times, even when you know the subject has seen you, you can snap away unhindered. It is even, at times, as though the subject is enjoying the attention. Or perhaps, they feel too awkward to respond and complain.
In real life, most people, I believe, would enjoy seeing images of themselves. Even at times when we feel mightily unsettled, there is something special about having a record of your image to look at in future times. Images can be very emotive – reminding you of times, people and places.
But, some people can only feel this if they re in control of the process – if they give permission and if they invite the photographer in. Candids, of the type described above, means that the photographer is in control – in control of the taking of the image AND its disposal.