The Bystander Effect is a theory of behavior that contends that when a lot of people are involved in witnessing a situation taking place they are less likely to get involved under the contention that someone else in the crowd may do it. However, if it were a situation where 2 or 3 people are involved, and someone needs to take action (e.g. defending someone or making a statement) then it is more likely that one of the members of the small group will do it, precisely because there are not enough people to make a decision and someone's got to do it.

In Chronicles of a Death Foretold, like the title itself implies, the situation that was going to take place was already assumed or known by many people, if not by everybody. However, nobody bothered to tell Santiago anything of value that would have changed the way things turned out.

Instead, the attitude was of saying "I thought it was a joke" or "I did not know" and yet, everyone was talking about it. Like the narrator says:

There had never been a death more foretold

However, the most telling quote, which lays the guilt equally among everyone in the village is in chapter 1 where it says:

No one ... wondered whether Santiago Nasar had been warned, because it seemed impossible to all that he hadn't

That quote epitomizes the Bystander Effect: the collective attitude of thinking that if everyone knows something is going on, surely someone else would have already told him, so why get involved?

This silence ultimately helps propel the chain of events that leads to the death of Santiago who, ironically, is completely unaware of how bad his situation really is.