Field interviews or interrogations are procedures that could fit in well with community policing approaches to crime prevention. The basic idea behind field interviews is that police officers should stop and interview anyone who seems to be acting in a suspicious way. This can prevent crimes from being committed either by those particular people or by others who see the interviews going on and conclude that police presence is strong and they will likely be caught if they commit a crime.
This means that the situations you are asking about are those in which a person (or a group) is acting in a suspicious way. One such situation could be if a few people are standing around outside a store that is not open (assuming there is nothing else around the store that would give the group a good reason for being there), particularly at night. The officer might well suspect that the group is thinking of breaking in to the store.
A second situation might be one in which someone is standing on the sidewalk or sitting on a porch and frequently approaching cars that stop near him. This sort of behavior is not really typical of any innocent activity and might indicate that the person is dealing drugs.
In situations such as these, it would be reasonable for a police officer to initiate a field interview.