Reading 192, from Gale Enclopedia

In all cases of voluntary omission, whether due to
nonchoice or deliberate choice, one is responsible for his
failure to act, and the morality of the omission is determined
in accord with the same norms that are applied to
a positive choice to do something. If there is advertence
to the obligation to do something, the omission of the performance
of the act is equatable with a positive sinful
choice, and the result is a grave or venial sin as the gravity
of the obligation and the circumstances of the case demand.
It is to be noted, however, that choices of
nonperformance are qualified by the same influences that
affect choices to act. Accordingly, such subjective elements
as fear and passion can, under certain conditions,
lessen personal imputability.

Gale Group’s New Catholic Enclopedia, vol 10

My notes:

In simple terms, the failure to act when action is needed is as good as choosing evil over good. So beware of sitting silent when you could have made a difference.