The Meaning of Candid Shots
Being prepared is essential - carry a camera at all times and have it ready for use.
Since the opportunity for shots may occur when the light is low and there is no time for flash, or when flash might be intrusive, try a medium-fast film such as 200 or 400 ISO.
If you do take a photograph quickly and the subject is still in position, see if you can shoot from a different viewpoint that may be better.
Even if the person moves away altogether you will still have that important first shot, but chances for improvement are usually available.
Perhaps the best lens to use would be a 80-300mm zoom lens.
This means that the picture can be framed in such a way as to crop out any unwanted detail.
It also means that you can get in close to the subject from some distance away without having to change lenses.
In this way you can work unnoticed without inhibiting the person or people and ruining the spontaneity of the situation.
In candid photography it is often the look, gesture or position of a person that makes the shot.
It is an advantage to have TTL metering, preferably with spot metering facilities.
This means a reading can be taken from the subject's face and you can expose for the skin tone.
If the person is against a white wall or bright reflective background then there is the risk of underexposure if the meter takes an average reading; this is because the background would be giving off the most light.