What is the doctrine of Novus Actus Interveniens, and why is it so important?

Novus actus interveniens, is a Latin phrase which translates to mean: an intervening act.
For the actus reus of the crime to be established, the defendant must be both the factual and legal cause of that crime. The defendant will be the legal cause of the consequence if his conduct was the 'operating and substantial cause' of that result (Smith 1959).
Novus actus interveniens is important, as this is when an act (either an act of a third party, an act of the victim or an act of 'god') breaks the chain of causation and means that the defendant is no longer the substantial and operating cause, so cannot be the legal causation of that crime.